Total knee arthroplasty (TKA) has evolved into a successful operation with acceptable longevity for patients with incapacitating arthritis of the knee. Over the years, the success of this procedure has been measured mainly by means of survivorship analysis.19 Survivorship measures durability but does not reflect the functional results of the procedure. Despite the introduction of functional parameters in the Knee Society rating system,15 we believe the information on the long-term functional outcome of TKA is inadequate. The function score of the Knee Society rating system has several limitations. It only includes gait and the ability to go up and down stairs, ignoring a number of daily activities of importance to the patient.15 In addition, it uses a scoring system that has a ceiling effect because of the linear relationship between walking capacity and the number of points allocated. Consequently, all patients who are able to walk 2 km and go up and down stairs are positioned within the good or excellent group without further stratification. Lastly, there is insufficient information as to what matters for the patient in terms of functional capacity.30
Another reason for the apparent lack of information on functional outcomes is validated functional outcome scores often are not used in traditional followup studies. A structured literature search from 1995 to April 2003 of English-language databases on functional outcomes concluded that, “TKA is a generally effective procedure, but the current English-language literature does not support specific recommendations about which patients are most likely to benefit from it”.16 However, a recent study by Noble et al20 looked specifically at functional activities that play an important role in daily life, and compared 243 patients with TKA versus 257 individuals (matched for age and gender) without previous knee disorders. They concluded activities that placed few demands on the knee in terms of muscle control or range of motion were equally important to both groups. More demanding activities, such as squatting, dancing, carrying heavy objects, kneeling, and turning were avoided by patients with TKA.20 Based on the experience in the outpatient clinic and the description of the activities, we assume four main factors play a role in functional deficits: range of motion, intrinsic stability, strength, and proprioception.30 Although the original arthritic disease accounts for part of the deficit in the form of irreversible sequelae, the kinematic abnormality of the replaced knee accounts for the other part.
Of all functional parameters, range of motion (ROM) is most often quoted by patients and orthopaedic surgeons. Reliable information on ROM can be found in fluoroscopic studies2-4,9-11,25,26,28 that measure motion in an accurate and reproducible way. Fluoroscopy in multicenter studies suggest a substantial limitation of the weightbearing ROM of the replaced knee with typical values between 100° and 110°.3,9,11 Because these studies include many different surgeons and implant designs, it may be assumed this is a limitation of our current technology. There is an ongoing discussion as to whether the surgical trauma or the prosthesis design plays a role in this functional deficit.
We explore evidence for the hypothesis that more normal kinematics will lead to better flexion in the replaced knee. Results of studies relating dynamic in vivo kinematic features against a control group of posterior stabilized TKAs were analyzed. The characteristics allowing flexion in the physiologic knee joint were compared to the kinematic patterns in the replaced knee. We analyzed kinematic abnormalities and related these to current limitations of total knee arthroplasty.
Flexion of the Normal Knee
Flexion of the human knee occurs along the 6° of freedom in space and includes rotation along the horizontal axis (flexion), translation along the sagittal axis (roll-back of the femur) and rotation over the coronal axis (femoral external rotation). This concept of coupled motion is a sine qua non for a physiologic flexion arc.8 The motion is guided mainly by the cruciate ligaments and the surface geometry of the tibial plateau with its concave and stable medial side versus a convex and sloped lateral side (Fig 1). The roll-back is greater on the lateral side than on the medial side.10,14 On the lateral side, this clears the back of the knee in deep flexion.14 On the medial side, the greater condylar offset allows the posterior femur to accommodate the posteromedial tibia without impingement.4 The rotational behavior in the horizontal plane also affects patellofemoral tracking. The internal rotation of the tibia relative to the femur reduces the Q-angle with increasing flexion, thereby stabilizing the patella.
Flexion of the Replaced Knee
As in the normal, physiologic knee, the motion is guided by the surface geometry. The cruciate ligament system is absent because the anterior cruciate ligament (ACL) is almost always sacrificed. If the posterior cruciate ligament (PCL) is retained, it can offer a constraint against sagging of the tibia, but in the absence of its anterior counterpart it cannot guide the motion as in the normal knee.25 Other systems substitute the PCL through interaction between the polyethylene insert and the femoral component: the cam-post mechanism. None of the current systems substitute for the ACL; therefore, the guidance of the motion is limited to a forced roll-back in flexion in those so-called “posterior stabilized systems.” In addition, these systems are traditionally designed for symmetric roll-back, giving equal posterior translations on the medial and lateral sides. If the knee follows its normal rotational pattern in the horizontal plane, with asymmetric femoral roll-back, this can cause a cam-post conflict and post wear.21 Kinematic patterns of the replaced knee have been well documented in fluoroscopic in vivo studies.2-4,9-11,22,25,26,28 Some patterns can be related directly to an important functional parameter that can be measured as a hard endpoint: flexion of the replaced knee.
Posterior condylar offset
The posterior condylar offset has been described as a determinant for flexion.4 Based on the analysis of 150 consecutive patients with PCL-retaining TKA, a correlation between the operative restoration of the posterior condylar offset and the maximum flexion of the knee was shown. A mean reduction in flexion of 12° was found with every 2 mm decrease in offset. This phenomenon can be explained by inadequate clearance in the back of the knee, with resulting impingement of the posterior polyethylene rim against the back of the femur. The posterior soft tissues can be caught between, leading to posterior knee pain. The restoration of this offset relates directly to surgical technique and prosthesis design. Anterior referencing with downsizing of the femoral component is a common cause and should be avoided. The authors recommend posterior referencing with accurate restoration of the posterior condylar offset. This surgical technique has a trade-off: in order to accommodate the front of the femur without running the risk of notching or overhang, many femoral sizes of the implant are needed with intervals of 2 to 3 mm in the sagittal plane.
The posterior translation of the femur relative to the tibia is another determinant of flexion in the replaced knee. In synergy with the restoration of the posterior condylar offset, femoral roll-back helps to clear the posterior aspect of the knee, especially on the lateral side.4,10,11,28 In addition, it has been shown femoral roll-back increases the lever arm of the quadriceps muscles at a critical phase of the stair-climbing cycle and provides increased mechanical advantage for the quadriceps muscle to extend the knee.1 In a prospective randomized trial comparing cruciate-retaining versus cruciate-substituting TKA, a difference in kinematic behavior was observed.28 Forward sliding of the femur during flexion, the so-called paradoxical motion, occurred mainly in cruciate-retaining knees and more on the medial than on the lateral side. Similar findings were reported in a matched pairs study comparing cruciate-retaining and posterior stabilized TKAs27 and in a multicenter summation analysis.10
Forward sliding of the femur during flexion can be attributed to several mechanisms. In extension, the starting position of the femur may be overly posterior in relation to the tibia (Fig 2). Near full extension, the quadriceps pull has a forward-directed vector component that is countered in the normal knee by the ACL. Absence of the ACL allows the tibia to slide forward, putting the femur in a too posterior “starting position.” During the flexion arc, forward sliding of the femur will occur as a compensatory mechanism. This phenomenon is observed in ACL-deficient knees and in all TKAs that sacrifice the ACL. Second, in insufficient PCL function of the cruciate-retaining knee, the tibia is pulled backward by the co-contraction of the hamstrings, allowing the femur to continue its forward slide. This phenomenon is only observed in cruciate-retaining knees (not in posterior stabilized knees) because the cam-post mechanism prevents this.28 A correlation between the anteroposterior positions of the femur relative to the tibia was observed. The replaced knee lost 1.3° of flexion with each millimeter of forward sliding of the femur. This observation is found in several other studies with similar magnitudes in different designs.3,4 It is a confirmation of the relationship between kinematic pattern and the outcome of a functional parameter.
Femoral external rotation
Abnormal rotational behavior has been described in several papers dealing with fluoroscopic analysis of the replaced knee.2-4,9-11,22,25,26,28 The abnormal pattern is more outspoken in cruciate-retaining knees than in posterior-stabilized knees. The center of rotation in the horizontal plane is on the medial side in the normal knee, whereas it is highly variable in the replaced knee.28 To our knowledge, no study is available specifically relating functional outcome with axial rotation between femur and tibia during flexion. A theoretical advantage for the patellofemoral mechanism is apparent. Femoral external rotation during flexion reduces the Q-angle and patellar shear force, and patellofemoral joint reaction force decreases. All modern posterior stabilized knee designs induce femoral roll-back equally on the medial and lateral sides. This mechanism should deliver a center of rotation in the horizontal plane in the middle of the knee. That kinematic studies on posterior stabilized knees show a more medial center of rotation3,10,11,22,26,28 can be explained by the isometric nature of the medial collateral ligament that will not allow too much roll-back on the medial side and forces the replaced knee more in the direction of the original kinematic pattern. A second disadvantage of symmetric roll-back is a potential conflict in the front of the knee. Impingement of the patellar tendon with the front of the polyethylene insert and contact between the patella and the post have been described.27
As total knee arthroplasty is a surface replacement within the existing soft tissues sleeve, it functions within normal anatomic and physiologic boundaries. Impaired functionality after TKA is attributed to sequelae of the arthritic disease, the surgical trauma and the design of the prosthesis. Recent information on the outcome of minimally invasive procedures suggests the reduction of the surgical trauma offers early improvement and faster rehabilitation.13,17,18 This effect levels off after 3 months to a result similar to that in patients who had a standard exposure.13,17 This means factors other than exposure and extensor mechanism violation are involved in the reduced functionality after TKA.
Aberrant kinematics including abnormal tibiofemoral rotation in the horizontal plane and forward sliding of the femur on the tibia, the so-called paradoxical motion, have been demonstrated in TKA patients.2-4,9-11,22,25,26,28 Forward sliding of the femur on the tibia negatively affects strength because of a shorter moment arm between the tibiofemoral joint reaction force and the force exerted by the patellar tendon on the tibial tuberosity. The relationship between kinematic behavior and knee flexion is the second link to postoperative function. A direct relationship between a kinematic parameter and the functional outcome is hard to confirm since few papers have related kinematic features to clinical outcomes. A second limitation of current kinematic studies using in vivo fluoroscopic analysis is they typically study small, selected groups of patients, and no control group. Also, the typical activities studied (gait, deep knee bend, step-up) do not necessarily represent the spectrum of kinematic conditions patients routinely perform. In one randomized controlled trial comparing between CR and PS knees, the fluoroscopic analysis was directed to a subgroup of patients.28 Other studies have reported matched pairs3,11,22,26 in comparing CR and PS knees. All studies3,11,22,28 but one26 related the more natural kinematics of the PS group to better flexion. In one paper, the number of investigated patients was high enough (40) to show statistical significance.11 The highest number of patients was enrolled in a multicenter summation analysis.10 The authors refer to the smaller magnitudes of femoral rollback during deep flexion as a main reason why knee flexion is reduced after TKA. One group compared fixed and mobile bearing PS knees and found normal axial patterns of rotation but decreased femoral rollback as compared to the normal knee.22 Maximum weightbearing flexion was not mentioned.
The comparison of kinematic data on the replaced knee with the characteristics of the normal knee supports the rationale for a knee implant that envisions a better kinematic performance: physiologic rotation in the horizontal plane, anatomic posterior offset and physiologic roll back. Differential medial and lateral surface geometry (Fig 3A, B) and guidance by the cruciate ligaments or an intrinsic asymmetric cam-post mechanism (Fig 3C) can provide this (Ries MD, Victor J, Bellemans J, Otto J, McKinnon B, Parikh A, Sprague J, Salehi A. Effect of guided knee motion and high flexion TKA on kinematics, implant stresses and wear. Proceedings of the AAOS 2006 Annual Meeting, Chicago Illinois, 2006).23 Preserving the anterior and posterior cruciate ligament has been attempted and resulted in more normal kinematics and better flexion.2,7,25 Surgical feasibility limits the potential of this solution.
The other solution is to provide kinematic guidance and stabilization by the prosthetic design through surface geometry and the cam-post mechanism.23 Some authors fear intrinsic motion guidance and a more anatomic tibial geometry could lead to accelerated wear.12 These authors generally prefer the original symmetric toroidal design with its inherent stability, surgical reproducibility, and wear characteristics. They suggest as long as designs do not try to reproduce normality in kinematics, wear will not be an issue.12 As polyethylene wear should be a primary concern with every new development in knee arthroplasty, it is worth taking a closer look at the multifactorial process leading to failure. First, osteolysis in TKA occurred with the advent of modularity in total knees.29 Backside wear was recognized as a main source of particles in the replaced knee. Improvements have since been made and the solution of a monobloc tibial component is not incompatible with a more kinematic knee design. Second, it has been recognized multidirectional sliding is detrimental for polyethylene.5 However, a guided kinematic pattern cannot be compared to the uncontrolled multidirectional sliding pattern of the unstable total knee; the sliding velocity is lower than in the unstable knee and the pattern of motion is unidirectional (Ries MD, Victor J, Bellemans J, Otto J, McKinnon B, Parikh A, Sprague J, Salehi A. Effect of guided knee motion and high flexion TKA on kinematics, implant stresses and wear. Proceedings of the AAOS 2006 Annual Meeting, Chicago, Illinois, 2006).
Third, the poor polyethylene performance in knee arthroplasty may be related to inadequate quality control in manufacturing, gamma-irradiation-in-air and shelf aging.6 Excellent long-term wear performance in a total knee design that offers little conformity but incorporates compression molded vacuum sterilized polyethylene supports the idea a more kinematic knee is possible from a material fatigue and wear standpoint.24 This is confirmed by recent work on implant stress and wear in a kinematic knee design, using dynamic finite element analysis and a virtual Oxford Rig (Ries MD, Victor J, Bellemans J, Otto J, McKinnon B, Parikh A, Sprague J, Salehi A. Effect of guided knee motion and high flexion TKA on kinematics, implant stresses and wear. Proceedings of the AAOS 2006 Annual Meeting, Chicago, Illinois, 2006).
Numerous in vivo fluoroscopic studies in patients who had TKA have shown abnormal kinematic patterns as compared to the normal human knee.2-4,9-11,22,25,26,28
Some factors have been correlated with inferior function after TKA. There is emerging evidence achievement of better kinematic patterns will help the patients in their functional performance. As the role of the kinematics is recognized, it can be expected new surgical techniques and implant designs will be developed to better serve the functional needs of the patient. The main caveats are surgical feasibility of performing the operation in a reproducible way and with durable materials. Close clinical followup and better functional monitoring will be needed to support the hypothesis of better functionality through kinematic normality.
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