Total Knee Arthroplasty - part 2 doc

28 I Essentials Relationship of Alignment to Kinematic Function of the Knee Kinematic function of both the normal and the replaced knee is the subject of other chapters, but the relationship of ligament function and component placement to alignment parameters must be understood in both the normal and the replacement scenarios Kapandji has best illustrated these concepts [5] The collateral and cruciate ligaments of the knee function normally only when they bear a normal relationship to the anatomy of the normal knee The axes around which flexion/extension occurs encompass the epicondyles, which are located within the concavity of a line connecting the instant centers of rotation of the knee (⊡ Fig 4-7) Because of this location, the collaterals are taut in extension and become relaxed as flexion proceeds This relaxation is a function of both the position of attachment and the posterior slope of the tibial plateaus Imagine that the attachment point of the MCL were picked up and physically moved anterior to its anatomical attachment (⊡ Fig 4-8) It can now be seen that the ligament would be tightened in flexion, a condition that would actually block flexion In the natural knee the articular surfaces cannot practically be repositioned, but in total knee replacement it is very easy to so In other words, it should be possible to perfectly align the components with the alignment references that have been outlined above and thereby maintain the relationship of the ligaments to the articulating surfaces of the new construct Alignment references for the normal knee consist of rotational alignment around the x-, y-, and z-axes However, in TKR, alignment also includes position along the alignment axes ⊡ Table 4-1 outlines all of the alignment parameters that are important to TKR It is only when all of these parameters are addressed and successfully fulfilled that a total knee replacement can function in a kinematically normal way If the surgeon is willing to use a totally constrained prosthesis, most of the alignment parameters can be ignored Varus/valgus alignment has an obvious cosmetic component and would not be ignored, nor would flexion/extension alignment The less constrained the prosthesis, the more important the align- ⊡ Table 4-1 All of the alignment parameters that are important to total knee replacement X-axis Y-axis Z-axis Malalignment around axis Flexion/ extension Internal/ external rotation Varus/ valgus Malalignment along axis Medial/ lateral translation Proximal/ distal displacement Anterior/ posterior displacement a b ⊡ Fig 4-7a, b (a) Medial view, (b) lateral view The attachments of the collateral ligaments to the epicondyles lie within the line connecting the instant centers of rotation Because of this anatomical location, and because of the decreasing radii of curvature of the condyles and the posterior slope of the plateaus, the ligaments are under less tension in flexion than in extension (After Kapandji) a b ⊡ Fig 4-8a, b The attachment of the medial collateral ligament has been physically moved anterior to the line connecting the instant centers In flexion, the attachment points are becoming further apart With continuing flexion, such a phenomenon would either block flexion or stretch out the ligament, which would produce instability ment Whenever prosthetic constraint is substituted for alignment and ligament balance, stress is transferred to the interfaces and to the prosthetic components Posterior cruciate substituting prostheses will be less sensitive to flexion instability caused by malalignment, but ignoring flexion stability will produce post wear, and even post fractures have been reported [7] Alignment Issues in TKR From the beginning of the history of TKR, alignment has been oversimplified Interest has focused mainly on varus/valgus alignment, which is mostly what the patient sees.However,the femur can be perfectly aligned with the tibia, and the components can be even severely 29 Chapter · Alignment of the Normal Knee – D S Hungerford, M W Hungerford 90° ⊡ Fig 4-9 Although this patient’s leg is neutrally aligned and the mechanical axis passes through the center of the prosthesis, the femoral component is displaced anteriorly, producing instability in flexion and leading to the dislocation seen here malaligned (⊡ Fig 4-9) In fact, the most common form of TKR alignment, introduced by Freeman [2] and Insall [4] in the late 1960s and early 1970s, produces minor offsetting malalignments of the femoral and tibial components This has been referred to as the ‘classic’ alignment method as opposed to the ‘anatomical’ alignment method introduced by Hungerford, Kenna, and Krackow [3] The ‘classic’ method makes tibial and femoral cuts to place the joint line perpendicular to the mechanical axis However, from Fig it can be seen that the normal joint line is not perpendicular to the mechanical axis The classic alignment therefore produces a 3° varus malalignment of the femoral component that is offset by a 3° valgus malalignment of the tibial component These produce a balanced knee in full extension However the valgus cut on the tibia over-resects the lateral tibial plateau,and this produces lateral instability in flexion (⊡ Fig 4-10) Most systems using the classic alignment system recommend externally rotating the femoral component to compensate for this lateral instability in flexion [11] Romero et al compared the consequences of the two alignment systems in normal cadaver knees and found that both systems produce indistinguishable ligament balance throughout the whole range of flexion [10] There is one particular advantage of the classic system The tibial cut is perpendicular to the mechanical axis, and therefore a stem attached at 90° to the tibial base plate is lined up with the medullary canal.A long stem attached to a base plate used for an anatomical cut would have to be at 87° to the base plate, and this would necessitate separate components for right and left knees This is not an issue for most primary knees, since a 90° standardlength stem is easily accommodated within the metaphysis (⊡ Fig 4-11) ⊡ Fig 4-10 The tibial resection line is perpendicular to the mechanical axis in the ‘classic’ alignment method of Insall and Freeman, producing over-resection of the lateral plateau To avoid lateral instability in flexion, the femur must be externally rotated, producing a compensatory sunder-resection of the lateral posterior femoral condyle ⊡ Fig 4-11 Radiograph of a total knee implanted with anatomical alignment references The short stem points toward the lateral cortex but is easily accommodated within the tibial metaphysis 30 I Essentials ⊡ Fig 4-12 Measured resection resects that amount of bone that will reestablish the level of the articular surfaces at their original pre-disease level One intact surface is necessary as a reference for the resection level Measured Resection The concept of measured resection was introduced by Hungerford, Kenna, and Krackow in 1978 [3] and is currently incorporated to some degree in most instrument systems The concept involves resecting that amount of the distal and posterior femur that will be replaced by the prosthetic components (⊡ Fig 4-12) Following this concept will place the articular surfaces of the replaced knee at the same level as they were in the natural knee This is usually possible for a primary total knee replacement,because there is generally at least one intact reference point for both the distal and posterior joint lines If a normal knee is replaced in this way, the replaced knee functions kinematically identical to the normal knee Of course, one argument could be that the knee that is a candidate for replacement is not a ‘normal’ knee However,most of the kinematic abnormalities that afflict the arthritic knee are due to lost cartilage and bone that takes place during the arthritic process This loss will be replaced through the proper implantation of the prosthetic components and kinematic balance will be restored The ultimate goal of TKR must include both normal alignment and ligament balance One without the other is unacceptable Martin and Whiteside have shown that it is possible to achieve ligament stability in both full extension and 90° of flexion in spite of malpositioning the femoral component proximally and an equal amount anteriorly (theoretically offsetting malalignments) and using a correspondingly thicker tibial spacer [6] However, doing so produces mid-flexion instability Consequences of Malalignment Malalignment has four basic consequences, three due to the overload conditions that are imposed Interface over- load produces aseptic loosening Plastic overload accelerates wear.Ligament overload produces pain and/or limits motion.Malalignment may also produce instability.Of the 275 revision total knee replacements performed at the Good Samaritan Hospital in Baltimore between 1983 and 1993, one or more malalignments contributing to failure were identified [8] A comprehensive review of the subject of malalignment is beyond the scope of this chapter.However,the importance of the subject to the success of TKR can be illustrated by dissecting the cause of an undesirable finding at the time of trial reduction: lateral patellar subluxation The ‘knee-jerk’ response to such a finding is to perform a lateral retinacular release and move on However, unless the patella was subluxing prior to the arthroplasty, something was done during the arthroplasty that has produced this condition, and that ‘something’ should be discovered and corrected Reasons for Patellar Subluxation There are nine malalignments that produce patellar subluxation: ▬ Femoral component malalignment This comprises internal rotation, medial displacement, and valgus malalignment These three reorient, or displace the trochlear grove to increase the ‘Q’angle,increasing the tendency toward lateral patellar subluxation ▬ Anterior displacement/femoral component oversizing: These both displace the trochlea, and hence the patella, anteriorly, tightening the lateral retinaculum and increasing the tendency toward lateral subluxation ▬ Tibial component malalignment This comprises internal rotation, medial displacement, and valgus malalignment.These three displace the tibial tubercle 31 Chapter · Alignment of the Normal Knee – D S Hungerford, M W Hungerford laterally, increasing the ‘Q’ angle, and increasing the tendency toward lateral patellar subluxation ▬ Patellar component malalignment: a) Under-resection of the patella displaces the ligament attachment to the patella more anteriorly, tightening the lateral retinaculum and increasing the tendency to lateral subluxation b) Lateral displacement of the patellar component laterally displaces the center of the patellar articulating surface, requiring medial translation to interface with the trochlea This increases the ‘Q’ angle and increases the tendency to subluxation There is no patellar subluxation in the majority of the knees presenting for replacement Therefore, if there is patellar subluxation at the end of the procedure,it is more logical to look for a cause rather than simply jump to a lateral retinacular release Similar circumstances apply to fixed flexion contracture,medial-lateral instability or imbalance,instability in flexion,instability in extension,global instability,and limited flexion All of the above can be associated with the presurgical pathology, or all of them can be produced by component malalignment It is the surgeon’s responsibility to eliminate these adverse conditions prior to closing the knee, and in order to so he/she must understand the origins of the problems, including the possible role of malalignment Significant malalignment is usually revealed during the trial reduction by imposing the abnormal kinematics that are characteristic of it References Brantigan OC, Voshell AF (1941) The mechanics of the ligaments and menisci of the knee joint J Bone Joint Surg 23:44–66 Freeman MA, Swanson SA, Todd RC (1973) Total replacement of the knee using the Freeman-Swanson knee prosthesis Clin Orthop 94:153–170 Hungerford DS, Kenna RV, Krackow KA (1982) The porous-coated anatomic total knee Orthop Clin North Am 13:103 0150122 Insall J, Ranawat CS, Scott WN, Walker P.Insall J, Ranawat CS, Scott WN, Walker P (1976) Total condylar knee replacment: preliminary report Clin Orthop120:149–154 Kapandji IA (1990) The physiology of the joints, vol II Churchill Livingstone, New York Martin JW, Whiteside LA (1990) The influence of joint line position on knee stability after condylar knee arthroplasty Clin Orthop 259:146–156 Mauerhan DR J (2003) Arthroplasty Fracture of the polyethylene tibial post in a posterior cruciate-substituting total knee arthroplasty mimicking patellar clunk syndrome: a report of cases J Arthroplasty 18:942–945 Mont MA, Fairbank AC, Yammamoto V, Krackow KA, Hungerford DS (1995) Radiographic characterization of aseptically loosened cementless total knee replacement Clin Orthop 321:73–78 Moreland JR, Bassett LW, Hanker GJ (1987) Radiographic analysis of the axial alignment of the lower extremity J Bone Joint Surg [Am] 69:745–749 10 Romero J, Duronio JF, Sohrabi A, Alexander N, MacWilliams BA, Jones LC, Hungerford DS (2002) Varus and valgus flexion laxity of total knee alignment methods in loaded cadaveric knees Clin Orthop 394:243–253 11 Worland RL, Jessup DE, Vazquez-Vela Johnson G, Alemparte JA, Tanaka S, Rex FS, Keenan J (2002) The effect of femoral component rotation and asymmetry in total knee replacements Orthopedics 25:1045–1048 5 Functional In Vivo Kinematic Analysis of the Normal Knee A Williams, C Phillips Summary The concept of tibiofemoral “roll-back”driven by tension in the cruciate ligaments (the “four-bar linkage” theory) as a model of tibiofemoral motion during knee flexion has dominated thinking for the past 30 years Some obvious flaws have been overlooked, however An interventional MRI scanner has been used to allow study, for the first time, of the weight-bearing living knee during a squat,in three dimensions.Results show that during knee flexion the lateral femoral condyle does move posteriorly, whereas in the active range of flexion the medial femoral condyle does not move significantly This differential motion equates to femoral external rotation (or tibial internal rotation) It is proposed that this axial rotation is driven by the shapes of the articular surfaces, and not the ligaments The findings have far-reaching implications for arthroplasty and the understanding of ligament function Introduction The biomechanics of the normal knee has been a subject of on-going speculation since 1836 Different theories as to how the tibia, femur, and patella articulate have developed as a result of research involving cadaveric and living subjects One of the biggest challenges still encountered is how to study functional kinematics of the knee, taking into consideration how muscle contraction,movement, and loading affect joint position which produces high-resolution images in any plane, thereby allowing accurate three-dimensional analysis of the knee joint However, due to the space constraint in conventional MRI scanners, studies have been nonweight bearing and involve a small range of knee motion “Interventional” Magnetic Resonance Imaging Although many different types of “open”scanner are regularly used in the clinical setting, few vertical-access “interventional” scanners exist worldwide One is based at St.Mary”s Hospital,London,UK.This model design incorporates a 0.5-T magnet housed in two vertical coils spaced 56 cm apart (⊡ Fig 5-1) Despite the magnet”s field strength being a third of that encountered in conventional scanners, the images produced are of satisfactory resolution, enabling dynamic analysis of bony and soft-tissue structures within the knee.As a result of the space,subjects can be scanned during active movement from full extension through to full flexion in both non-weight-bearing (seated) and physiological weight-bearing positions Methods of Investigating Knee Motion The majority of methods incorporate either invasive or irradiating techniques or sometimes both, therefore reducing acceptability to the volunteers being studied In addition there can be problems in analysis such as the phenomenon of “cross-talk” in Röntgen Stereophotogrammetric Analysis (RSA) [1] Magnetic Resonance Imaging (MRI) is an attractive tool, being a noninvasive technique that does not involve ionizing radiation and ⊡ Fig 5-1 0.5 Tesla interventional MR scanner 33 Chapter · Functional In Vivo Kinematic Analysis of the Normal Knee – A Williams, C Phillips FFC d ⊡ Fig 5-2 Scanning in non-weight-bearing position ⊡ Fig 5-4 Measurement of the position of the posterior femoral condyles relative to the tibia FFC, Flexion Facet Center; d, distance measured to ipsilateral posterior tibial cortex (after [2]) ⊡ Fig 5-3 Diagram of scanning in full weight-bearing position This scanner design incorporates two methods of image registration, known as “Flashpoint Tracking” and “MR Tracking”,which allow images to be continually obtained from one chosen plane in the knee joint, irrespective of significant movement between consecutive scans Either of these “tracking”devices and a receiver coil are attached to the subject”s knee (⊡ Figs 5-2 and 5-3) This facility makes it easy to accurately assess relative movement of femur on tibia, during a full range of motion, while analyzing medial and lateral compartments simultaneously but individually.To achieve this,the position of the posterior femoral condyles relative to the tibia are measured in the sagittal plane at mid-medial and mid-lateral positions of the knee, according to the method of Iwaki et al [2] On individual scan images of the medial and lateral compartments in increasing increments of flexion, the centres of the posterior circular surfaces of the femoral condyles were identified and used as fixed femoral reference points [2,3].The distance between these and a vertical line drawn from the ipsilateral posterior tibial cortex was measured for each position with a Vernier caliper and corrected for magnification (⊡ Fig 5-4) Changes in this distance, with progressive increments of knee motion, thus represent relative motion of the femur on the tibia occurring with knee flexion Recent cadaveric studies have established the sagittal contours of the medial and lateral joint surfaces [1] Through several dynamic MR studies,the consequence of this articular geometry on knee kinematics has become apparent [3–5] Weight-bearing Tibiofemoral Motion Using Open-access MRI The use of this technique has produced some dramatic findings Through collaborative work, our findings have been compatible with results of other studies employing conventional MRI of cadaveric specimens [2, 6], horizontal access open MRI of the non-weight-bearing living knee [3, 7], and RSA [8] Primary results from the St Mary”s Interventional MRI Unit, analyzing weightbearing knees in living subjects,have now been reproduced in a number of studies [3–5,9].Knees have been scanned at 10° increments from hyperextension to 140° The results of the most detailed study of normal tibiofemoral motion are 34 I Essentials 35 30 25 Distance (mm) 20 FFC 15 ADJUSTED 10 5 -5 -10 10 20 30 40 50 a 60 70 80 90 10 11 120 130 140 150 Fle xion Angle ( egrees) d 35 30 Distance (mm) 25 20 FFC 15 ADJUSTED 10 -5 -10 b 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Flexion Angle (degr es) e ⊡ Fig 5-5a, b Mean AP translation of lateral (a) and medial (b) femoral condyles from extension to deep flexion summarized in the graphs of mid-medial and mid-lateral compartments [4] (⊡ Fig 5-5a, b) In the lateral compartment the femur moves posteriorly – fairly rapidly at first, then steadily until 120° (producing about 20 mm of displacement), and thereafter rather abruptly (a further 10 mm) into a deep squat Medially the situation is very different In the range of flexion to 120° there is little anteroposterior movement of the femur on the tibia, but from this point to full flexion there is a modest sharp posterior displacement akin to the lateral side (9 mm) The limit of active knee flexion is 120°, and the kinematics from here to a deep squat are a passive phenomenon and distinct from that occurring in earlier flexion The differential medial and lateral motion equates to longitudinal axial rotation with knee flexion; internal tibial rotation/external femoral rotation occurs around a medial axis.For an average-sized male knee this produces 20° of rotation Recent fluoroscopic studies have also confirmed this finding of longitudinal rotation with flexion [10].It is this axial rotation which, when viewed as a lateral projection of the knee fluoroscopically,gives the “illusion”of femoral “roll-back”, since the lateral femoral excursion, but not the medial, is appreciated at first glance Knee flexion can be divided into three arcs: the screwhome arc, the functional active arc, and the passive deepflexion arc Screw-Home Arc The screw-home arc is the movement of the knee between approximately 20° of flexion to terminal extension Little 35 Chapter · Functional In Vivo Kinematic Analysis of the Normal Knee – A Williams, C Phillips is known about this arc and its functional significance In contrast to the functional active arc there is profound asymmetry between the shapes of the medial and femoral condyles articulating with the tibia [1] (see below) The medial femoral condyle articulates with the upward sloping anterior tibial surface This contributes to the posterior part of the medial femoral condyle rising 1–2 mm with progressive terminal extension As the lateral femoral condyle rotates internally when it moves forward in extension, it rolls down over the anterior edge of the lateral tibial plateau to compress the anterior horn of the lateral meniscus; hence, presumably, the presence of a recess in the lateral tibial plateau and the sulcus terminalis of the lateral femoral condyle It is not yet known if the terminal rotation observed with screw-home is obligatory and it is the subject of on-going study Functional Active Arc The functional action arc from approximately 20° to 120° of flexion is influenced by neuromuscular control.During this phase longitudinal rotation with flexion is not obligatory and can, to a large extent, be reversed by voluntarily externally rotating the tibia during flexion, allowing the knee to function almost as a uniaxial hinge [3] Knee motion can vary within an “envelope” of kinematic boundaries [11] The mechanisms responsible for axial rotation with flexion are not defined and not appear to be simply under the control of the cruciate ligaments as was previously thought As well as voluntary control, the different shapes of the articulations are very important in this regard (see below) Passive Deep-Flexion Arc In the arc of 120º–140º of deep flexion,tibiofemoral motion is passive,as a result of external force (usually body weight) allowing extra flexion Medially the femoral condyle rises about mm as it moves posteriorly, riding up on the posterior horn of the medial meniscus This may explain why degenerate posterior horn tears of the medial meniscus often occur in deep flexion On the lateral side of the knee there is extreme movement of the lateral femoral condyle,which drops approximately mm as it nearly subluxes off the tibia Therefore, in a deep squat both medial and lateral condyles now move backwards close to subluxation, largely balanced, presumably, by extensor mechanism tension and posterior anatomical impingement Articular Contact Points It is natural to assume at first that relative motions of the medial and lateral tibiofemoral articular surface contact points will “mirror” the motion of the bones in terms of direction and in magnitude [12] If the sagittal profiles of the femoral condyles were single radius curves (i.e., a circle) or “J”-shaped (closing helix) curves and the tibial surfaces flat, this would have to be true The situation for a circle would be analogous to the wheel of a car moving on the road: Whether sliding or rolling, the contact point would lie on a line perpendicular to the road passing through the center of the wheel Hence, as the wheel moved, so, correspondingly, would the contact point In the knee, however, the situation is different and the actual anatomy present “disassociates” the movements of the articular contact points and of the bones Through detailed study of cadaveric specimens the sagittal shapes of the medial and lateral joint surfaces have been established [1] The medial tibia is flat for its posterior half, leading anteriorly to an “up-slope”.With the well-fixed and therefore relatively immobile posterior horn of the medial meniscus [13], the distal articular surface is significantly concave, thereby stabilizing the femur Laterally the tibia presents a broadly convex surface to the femur In this much less stable arrangement the lateral meniscus is highly mobile [13] to provide important load sharing with the articular surfaces.The medial femoral condyle surface describes arcs of two circles The more anterior is shorter and has a larger radius than the posterior Laterally the anterior arc is very small or even absent, so that the articular surface is effectively described by the arc of a single circle (⊡ Fig 5-6a, b) In the lateral joint compartment, the femoral surface moves posteriorly by a combination of rolling and sliding and,akin to a wheel,takes the articular contact point back with it Medially the joint surface motion is almost exclusively by sliding (i.e., “spinning on the spot”), initially in the early part of flexion, about the center of the more anterior “extension facet center” and then from about 30°–40° about the center of the more posterior arc (the “flexion facet center”) The shift in position of the “active” center of rotation is quite abrupt.This shift is accompanied by a corresponding posterior change in position of joint surface contact (similar to the change in position of the lateral joint surface contact point), but not a posterior bodily transition of the femur [14] This phenomenon is possible only due to the shapes of the articulating surfaces Implications and Future Developments While caution is necessary in extrapolating these results of knee motion,observed in a controlled squat,to normal daily activities such as walking and running, the authors 36 I Essentials a b ⊡ Fig 5-6a, b Sagittal MRI images of the lateral (a) and medial (b) tibiofemoral joints showing the posterior (FFC, Flexion Facet Center) and anterior (EFC, Extension Facet Center) circular arcs of the femoral condyles believe the findings of differential compartment motion in the knee to be very important Primarily, the results challenge the popular concept of femoral “roll-back” It is reasonable to argue that “roll-back”exists laterally.Due to lack of anteroposterior translation medially, in the active range of flexion (up to 120°) this term is not appropriate for the bone itself However, what of the contact area? First, “rolling” cannot be sensibly applied to change in position of an area Second, there is no steady transfer of contact through knee flexion provided by “rolling”; rather, as the knee flexes, the medial femur spins only abruptly changing the center about which it rotates and so allowing a change in articular contact position This is certainly not the description of “roll-back” that has hitherto been popularized Furthermore, the kinematics presented here produce the perceived benefits of the “roll-back” model The posterior shift of joint contact and femoral external rotation with knee flexion increase the extensor mechanism lever arm Femoral external rotation allows avoidance of posterior bone impingement, thereby maximizing flexion and providing the further benefit of reducing the “Q angle”, so aiding patellar kinematics Dynamic MRI allows analysis of not only bony structures, but also of the ligaments Previous mathematical models suggested that, when taut, the cruciate ligaments act as a rigid four-bar link, guiding TF motion Imaging of knees with both intact and deficient anterior and posterior cruciate ligaments during the full range of flexion, in loaded and unloaded positions [9,15], makes it evident that the ligaments not tend to play a great role in guiding motion in normal physiological movement of the knee when taut,but rather during excessive application of force, such as that encountered during sporting activity The ACL assists in controlling the static weight-bearing tibiofemoral position in the lateral compartment and the PCL acts similarly in the medial compartment Nevertheless, neither ligament influences the extent of active motion during weight-bearing flexion of the knee [4] It would seem likely that the articular surface geometry is a more potent factor driving knee kinematics The dramatic differences in sagittal shapes of the medial and lateral compartments account for the similarly clear differences in medial and lateral kinematics Much of the interest in knee kinematics has been directed towards optimizing prosthetic design The history of knee replacement shows that improvements in implant performance were associated with the designs becoming closer in shape to the natural knee.Current designs have produced very successful functional outcomes 37 Chapter · Functional In Vivo Kinematic Analysis of the Normal Knee – A Williams, C Phillips in the 0°–90° range of flexion Most are designed to produce femoral “roll-back” either by preserving the PCL (PCR) or substituting it for the cam-post mechanisms common to the posterior stabilized (PS) designs Both types perform well,despite the argument that is raging for and against the two groups Only the PS designs produce femoral roll-back; in reality, the PCR designs have rather erratic motion, including paradoxical anterior sliding of the femur during flexion [16].Since no prosthesis,total or unicompartmental, reproduces normal joint geometry, none can rightly claim to restore normal joint kinematics This is not to say that they not perform well; many do, but not by restoration of normal kinematics Rather, their functional success lies in the fact that the changes they impose are well tolerated Application of our observed tibiofemoral kinematics might be useful, particularly in restoring physiological knee function, including flexion However, one must proceed with caution.A simplistic view would be that a prosthesis allowing external femoral rotation about a medial axis during knee flexion, so as to provide more normal kinematics, might produce better results However, although we not believe in the four-bar linkage model, there will be some price for sacrificing the cruciate ligaments, and at best the prosthetic articular surfaces in current designs remain far from normal This means that these designs probably will not confer any advantage over current standard total condylar designs Perhaps the next generation of total knee replacements will require articular surfaces shaped in the anatomical manner, to guide more physiological knee motion and achievement of higher levels of function Acknowledgements We thank the English Football Association/Professional Footballers Association for generously funding Miss Carol Phillips”post, and Professor W Gedroyc, MRCP, FRCR, Director of The Interventional MRI Unit and Consultant Radiologist, St Mary”s Hospital, London References Martelli S, Pinskerova V (2002) The shapes of the tibial and femoral articular surfaces in relation to tibiofemoral motion J Bone Joint Surg (Br) 84:607–613 Iwaki H, Pinskerova V, Freeman M (2000) Tibiofemoral movement 1: The shapes and relative movements of the femur and tibia in the unloaded cadaver knee J Bone Joint Surg (Br) 82:1189–1195 Hill PF, Vedi V, Williams A, et al (2000) Tibiofemoral movement 2: The loaded and unloaded living knee studied by MRI J Bone Joint Surg (Br) 82:1196–1198 Johal P et al (2004) Tibio-femoral movement in the living knee: an in-vivo study of weight-bearing and non-weight-bearing knee kinematics, using “interventional” MRI J Biomechanics (paper accepted; in preparation) Todo S, Kadoya Y, Miolanen T, et al (1999) Anteroposterior and rotational movement of femur during knee flexion Clin Orthop Rel Res 362: 162–170 Pinskerova V et al (2001)The shapes and relative motions of the femur in the unloaded cadever knee In: Insall JN, Scott WN (eds) Surgery of the knee, chap 10, 3rd edn Saunders, Philadelphia, pp 255–283 Nakagawa S, Kadoya Y, Todo S, et al (2000) Tibiofemoral movement 3: Full flexion in the living knee studied by MRI J Bone Joint Surg (Br) 82:1199–1200 Karrholm J, Brandsson S, Freeman M (2000) Tibiofemoral movement 4: Changes of axial tibial rotation caused by forced rotation at the weightbearing knee studied by RSA J Bone Joint Surg (Br) 82:46–48 Logan M, Williams A, Lavelle J, et al (2004) What really happens during the Lachmann test? A dynamic MRI analysis of tibiofemoral motion Am J Sports Med 32:369–375 10 Komistek R, Dennis D, Mahfouz M, et al (2003) In vivo fluoroscopic analysis of the normal knee Clin Orthop Rel Res 410:69–81 11 Blankevoort L, Huiskes R, De Lange A (1988) The envelope of passive knee joint motion J Biomech 21:705–720 12 Wretenberg P, Ramsey D, Nemeth G (2002) Tibiofemoral contact points relative to flexion angle measured with MRI Clin Biomech 17:477–485 13 Vedi V, Williams A, Tennant S, et al (1999): Meniscal movement: an in vivo study using dynamic MRI J Bone Joint Surg (Br) 181:37–41 14 Pinskerova V et al (2004) Does the femur roll back with flexion? J Bone Joint Surg [Br] 86:925–931 15 Logan M, Dunstan E, Robinson J, et al (2004)Tibiofemoral kinematics of the ACL deficient knee employing vertical access open interventional MRI Am J Sports Med 32:720–726 16 Komistek R, Scott R, Dennis D, et al (2002) In vivo comparison of femorotibial contact positions for press-fit PS and PCL retaining TKA J Arthroplasty 17:209–216 56 II Past Failures chemical mediators from these activated cells [12] Cellular mediators that have been studied and are thought to play a significant role in osteolysis are interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and prostaglandin E2 (PGE2) [13] Proliferation of these cellular mediators then leads to stimulation and differentiation of osteoclasts and inhibition of osteoblasts These factors work synergistically, ultimately leading to the dissolution of bone at the prosthetic interface, allowing for prosthetic micro motion that leads to further generation of wear debris [14] In vitro studies have demonstrated that many factors influence the biological response to wear debris, including the size,volume,surface chemistry,and material composition of the particles [2] Many investigators have noted that osteolysis is less common in TKA than in THA [12, 15, 26] In THA, adhesive and abrasive wear mechanisms dominate, resulting in the formation of high volumes of sub-micron particulate debris Conversely, in TKA, fatigue and delamination are the most common mechanisms of bearing surface damage These modes of failure produce wear debris particles that are larger than the wear particles observed around total hip replacements It has been hypothesized by Ayers and others that the more aggressive biological response seen with THA could be explained by the fact that sub-micron particles provide a greater stimulus to the macrophage to produce inflammatory mediators that result in osteolysis [2] References Ayers DC (2001) Maximizing ultra high molecular weight polyethylene 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Paxson RD, Tanner MG, Whiteside LA (1996) Effects of sterilization on wear in total knee arthroplasty Clin Orthop 331:164–171 34 Willert H, Semlitsch M (1977) Reactions of the articular capsule to wear products of artificial joint prostheses J Biomed Mater Res 11:157–164 35 Williams IR, Mayor MB, Collier JP (1998) The impact of sterilization method on wear in knee arthroplasty Clin Orthop 356:170–180 36 Won CH, Rohatgi S, Kraay MJ, Goldberg VM, Rimnac CM (2000) Effect of resin type and manufacturing method on wear of polyethylene tibial components Clin Orthop 376:161–171 37 Wright T, Rimnac C, Faris P, Bansal M (1988) Analysis of surface damage in retrieved carbon fiber-reinforced and plain polyethylene tibial components from posterior stabilized total knee replacements J Bone Joint Surg [Am] 70:1312–1319 38 Wright TM, Bartel DL (1986) The problem of surface damage in polyethylene total knee components Clin Orthop 205:67–74 57 Chapter · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A Rand 9 Failures in Patellar Replacement in Total Knee Arthroplasty J A Rand Summary Patellar failure after TKA is often multifactorial A careful assessment of patient factors, implant design and surgical technique must be performed If there are major problems with implant design or component positioning, revision of the entire arthroplasty may be necessary to correct the patellar failure and ensure a durable result Isolated revision of the patellofemoral joint for any reason must be approached cautiously, as a high failure rate is often encountered Introduction In a recent series of revision total knee arthroplasties (TKA), extensor mechanism problems comprised almost 12% of the reasons for reoperation [1] Reasons for failure in the patellofemoral joint are multifactorial and may be related to patient selection, implant design, surgical technique, or combinations of these factors Therefore, any discussion of patellar component failures must consider multiple potential reasons for failure Unfortunately, most studies of patellofemoral complications have not considered the importance of the tibiofemoral joint for the complication Anterior knee pain, patellar instability, fracture, loosening, wear, extensor mechanism rupture, and a variety of miscellaneous problems affecting the patella can adversely affect the results of a TKA The anatomy and kinematics of the patellofemoral joint are complex There is variability in the orientation of the patellar groove in both the coronal and transverse planes.The patellar groove is oriented approximately perpendicular to the epicondylar axis Since there are substantial individual variations in alignment and patellar tracking, the design of the femoral component needs to accommodate these variations The patella undergoes a medial shift in early flexion followed by a lateral shift in deep knee flexion beyond 90°.In deep flexion,the contact area on the patella moves distally on the lateral facet of the patella,resulting in a decrease in contact area.The contact areas and kinematics of the patella are altered in TKA and are affected by implant orientation and design of the femoral and patellar components Etiology Patient selection is an important variable influencing extensor mechanism complications Patellar complications are increased in patients with a diagnosis of patellofemoral arthritis, obesity, osteoporosis, valgus deformity, post-traumatic arthritis, and prior proximal tibial osteotomy (⊡ Fig 9-1) A diagnosis of osteoarthritis and obesity has been associated with an increased risk of patellar complications [2] In the presence of valgus deformity, varying degrees of lateral femoral condyle hypoplasia make rotational positioning of the femoral implant difficult In knees with ⊡ Fig 9-1 Merchant X-ray demonstrating patellar subluxation bilaterally with fracture on the left knee in a patient with preoperative patellofemoral arthritis 58 II Past Failures a b ⊡ Fig 9-2a, b CAT scans of (a) femoral and (b) tibial component with internal malrotation preoperative valgus, a lateral retinacular release was necessary in 102 of 134 knees to treat intraoperative patellar subluxation [3] The presence of patella infera following proximal tibial osteotomy or post-traumatic arthritis can result in impingement between the patella and the tibial component, resulting in pain or patellar instability The patient with severe osteoporosis is at risk of patellar fracture following patellar resurfacing Surgical technique is an important variable influencing patellar complications.A midvastus or subvastus surgical approach results in improved patellar tracking and less frequent need for a lateral retinacular release than does an anteromedial arthrotomy Using meta analysis, a lateral retinacular release was required in ten of 164 (6%) subvastus approaches compared with 31 of 172 (18%) medial parapatellar approaches [4–6] Femoral and tibial component position affect patellar alignment and complications Patellar complications are diminished by maintenance of the joint line and patellar height, lateral placement of the femoral component on the femur, medial placement of the patellar component on the patella, and posterior placement of the tibial component on the tibia.The femoral component should not be flexed on the femur, and the trochlear flange should be aligned with the anterior femoral cortex If the trochelar portion of the femoral component is prominent, the extensor mechanism will be displaced in an anterior direction,resulting in increased lateral retinacular tension.The result is a potential decrease in knee motion and possible patellar maltracking In severe cases, the patella may mechanically catch on the trochlear flange of the femoral component Any deviation of the femoral, tibial, or patellar components from these ideal locations can adversely affect patellar alignment, leading to patellar failure Internal malrotation of either the tibial or the femoral component will adversely affect patellar tracking (⊡ Fig 9-2) Alignment of the femoral component with the epicondylar axis or the AP axis appears to be best Femoral component rotation parallel to the epicondylar axis resulted in the most normal patellar tracking and decreased shear forces early in flexion [7].Rotating the femoral component either internal or external to the epicondylar axis adversely affected patellar tracking There is a close relationship between the femoral epicondylar axis and the patellar axis Placing the tibial component perpendicular to the epicondylar axis resulted in correct rotation in 73% of cases [8] In a study of 102 TKAs, there was a mean of 6.2° of internal rotation in the knees with anterior knee pain compared with 0.4° of external rotation in the control knees [9] In a comparison of 30 TKAs with patellar complications and 20 controls without, combined internal rotation of 1°–4° resulted in lateral patellar tracking and tilt,3°–8° patellar subluxation,and 7°–17° patellar dislocation or patellar failure [10] Reproduction of patellar thickness, correct size and position of the patellar component,and balance of the extensor mechanism are necessary for a satisfactory result A lateral retinacular release was required for 17% with medial compared with 46% with a central placement of the patellar implant [11] The amount of bone resected from the patella will affect patellar tracking and patellar strain Resection of excessive patellar bone can result in weakening of the patella, leading to fracture or implant fixation in poor-quality bone predisposing to loosening Thickening the patella at the time of resurfacing will tighten the lateral retinaculum,resulting in patellar tilt or subluxation If the patella is resurfaced, the original patellar thickness should be reproduced Asymmetric resurfacing of the patella should be avoided In a series of 59 Chapter · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A Rand ⊡ Fig 9-3 Merchant X-ray demonstrating lateral patellar tilt and subluxation in a TKA design with a shallow trochlear groove There is also some internal rotation of the femoral component 21 TKAs with asymmetric resurfacing of the patella, 11 knees were revised, recommended for revision, or had anterior knee pain [12] A lateral retinacular release for patellar maltracking is not innocuous Patellar complications occurred in14% of 540 knees with in comparison to 7% of 510 knees without a lateral retinacular release [13] Complications in the lateral release group consisted of patellar radiolucency in 11, patellar fracture in nine, hematoma in seven, extensor lag in seven, patellar instability in five, and patellar implant loosening in three [13] Implant design affects patellar alignment and patellar tracking A trochlear groove that is asymmetric or deep produces a decrease in shear and compressive force on the patella compared with a symmetric or shallow trochlear groove design [14] Patellar complications occurred in 15 of 148 TKAs with a shallow trochlear groove compared with one of 153 TKAs with a deep trochlear groove [15] (⊡ Fig 9-3) In a comparison of 150 TKAs performed with a standard design with 150 TKAs with a 3° external rotation built into the femoral component (resulting in a lateralized trochlear groove), the prevalence of lateral retinacular release was decreased from 14% to 5% and patellar maltracking from 12% to 5% [16] Therefore, selection of a femur with a deep, offset trochlear design is preferable to minimize patellar complications The patellar implant may be a central dome, offset dome, or anatomical in shape Failure of the patella due to wear and deformation of patellar components are observed with all polyethylene and metal-backed patellar component designs All polyethylene dome designs are susceptible to deformation, while metal-backed designs are susceptible to deformation of the polyethylene over the metal or to dissociation of the metal backing In a study of six different TKA designs, contact pressures in the patellofemoral joint at knee flexion angles greater than 60° exceeded the yield point of the polyethylene for all designs [17].Anatomically shaped patellar implants provide increased contact area over dome-shaped implants when aligned correctly but will have a decreased contact area with slight tilt or malrotation Eighteen of 75 anatomically shaped patellar implants had a complication [18].For this reason,a domeshaped patella is preferred to the anatomical shape, as it is more forgiving of minor malalignment A study of a two-peg anatomical, three-peg dome, and three-peg anatomical patellar component design found improved results with the three-peg designs [19] Either an inset or onlay patellar design may be used In an in vitro comparison of an inset to an onlay design, patellar strain was increased by 28% for the inset and 22% for the onlay design over the unresurfaced value [20] In a retrospective comparison of 135 resurfacing patellae with 116 inset patellae, patellar tilt, subluxation, and lateral retinacular release were less frequent with the inset than with the resurfacing design [21] Are extensor mechanism complications different in resurfaced and unresurfaced patellae of TKA? Controversy has surrounded the need for patellar resurfacing at the time of TKA This controversy arises from differing results which are clearly influenced by studies using TKA designs that not allow congruent tracking of the native patella and surgical techniques that led to patellar malalignment These results are further complicated by differing rates for reoperation that are influenced by the ease of resurfacing of the painful unresurfaced patella but not treatment of anterior knee pain in the resurfaced patella Selective resurfacing attempts to identify those individuals who will have an improved clinical result by resurfacing while avoiding the complications of unnecessary resurfacing The best data regarding the results of patellar resurfacing derive from randomized,prospective studies of patellar resurfacing Using meta analysis of nine randomized,prospective series,there were 518 resurfaced and 542 unresurfaced patellae followed for 2–10 years [22–30].Anterior knee pain was present in 38 (7.3%) of the resurfaced and 118 (21.8%) of the unresurfaced patellae Knee scores were similar in both groups Patellar complications occurred in 14 (2.7%), leading to reoperation in ten (1.9%) of the resurfaced patellae This is in contrast to patellar complications in 37 (6.8%) of unresurfaced patellae, leading to reoperation in 36 (6.6%) [22–30] If anterior knee pain persists in the unresurfaced patella, will pain be relieved by resurfacing? Using meta analysis of 60 knees with secondary patellar resurfacing, 36 (60%) were improved, 12 (20%) unchanged, and 12 (20%) worse after resurfacing [25–29, 31, 32] Therefore, selective patellar resurfacing may be the best approach considering patient demands, implant design, patellar articular cartilage, and intraoperative patellar alignment 60 II Past Failures Specific Complications and Failure Mechanisms Patellar instability may manifest as pain and weakness, intermittent giving way, or episodes of patellar dislocation Patellar tilt and subluxation are frequently encountered on routine radiographs of asymptomatic patients Why some patients with patellar malalignment are asymptomatic while others are symptomatic remains an enigma The etiology of patellar instability can be traced to patient selection,implant design,surgical technique,or trauma [33] Preoperative patellar subluxation, preoperative valgus, or patellofemoral arthritis have been correlated with an increased prevalence of lateral retinacular release and increased postoperative patellar malalignment [3] Implant designs that have a shallow trochlear groove, fixed axis of rotation, or unrestricted rotation have had a high prevalence of patellar subluxation and dislocation In a study of 289 TKAs using a design with a shallow trochlear groove and followed up, 14 knees required revision for patellar maltracking [32] Problems with surgical technique are a frequent reason for patellar instability Internal malrotation of the femoral or tibial component, excessive valgus knee alignment, lateral placement of a patellar component on the patella, medial translation of the femoral component on the femur,thickening the patella with resurfacing, or oversizing the femoral component onto the anterior surface of the femur may contribute to patellar instability Treatment of patellar instability must be directed at the etiology Implant malposition is best treated by component revision Soft-tissue imbalance should be treated by a proximal realignment consisting of a lateral retinacular release and vastus medialis advancement Although distal realignment of the tibial tuberosity can correct patellar instability, it creates a compensatory deformity, alters patellar kinematics, and may predispose to patellar tendon rupture Most published studies of treatment of patellar instability did not assess implant rotation, making interpretation of the results difficult In our own series, the etiology of patellar instability was failure to balance the extensor mechanism in nine knees , tibial component malrotation in four,atraumatic medial retinacular tears in four, quadriceps weakness in four, activity-related tears of the medial retinaculum in two, and a traumatic tear of the medial retinaculum in one knee [34] Recurrent subluxation occurred in four of 14 knees following proximal realignment, while two of nine knees sustained a rupture of the patellar tendon following combined proximal and distal realignment One of two knees treated by revision had a recurrent patellar subluxation Two knees developed deep infections Distal extensor mechanism realignment has been recommended using a tibial tubercle transfer as treatment for patellar instability However, as distal extensor mechanism realign- ⊡ Fig 9-4 Lateral X-ray demonstrating an asymptomatic patellar fracture with non-union ment alters knee kinematics and is associated with rupture of the patellar tendon in some patients, proximal realignment is the preferred technique in the absence of implant malposition Revision of the femoral and tibial components should be considered for those knees with component malposition Patellar fractures may present as anything from asymptomatic findings on routine radiographs to acute episodes with disruption of the extensor mechanism (⊡ Fig 9-4) In a series of 12 464 TKA from the Mayo Clinic there were 85 patellar fractures, for a prevalence of (0.68%) [35].The prevalence of fracture is greater in men than in women, in resurfaced than in unresurfaced patellae, and in revision than in primary TKA The etiology of patellar fracture includes trauma, avascularity, implant malalignment, obesity, excessive patellar bone resection, high activity level, large range of motion, inset patellar design, large central fixation lug,revision TKA,and osteoporosis The position of the tibial and femoral components affects the severity and results of patellar fractures In a series of 36 patellar fractures, a good or excellent result was achieved in 15 of 16 with minor malalignment as compared with three of 20 with major malalignment [36] Avascularity of the patella following a medial arthrotomy combined with a lateral retinacular release has been suggested as an etiology of fracture.Another explanation for the association of patellar fracture with lateral retinacular release is abnormal forces on a maltracking patella that necessitated the lateral release In a series of 1146 TKAs,a patellar fracture occurred in 22 of 406 (5.4%) knees with as compared with 18 of 740 (2.4%) without a lateral retinacular release [37] There was no significant difference in the prevalence of patellar fracture following lateral retinacular release in which the superior lateral genicular artery was spared or sacrificed [38] A variety of classification systems have been used for patellar fractures after TKA Differences in classification make comparison of results from various series difficult 61 Chapter · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A Rand The easiest classification system is the one from the Mayo Clinic [35].Fractures are divided into three types based on the stability of the patellar prosthesis and on whether or not the extensor mechanism is intact A type-I fracture has a stable implant and an intact extensor mechanism.A type-II fracture has a disruption of the extensor mechanism.A type-III fracture has a loose patellar implant and an intact extensor mechanism The type-III fractures are divided into subtypes A and B, based on the quality of remaining bone stock (A = good and B = poor with