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Kinematics of the Stiff Total Knee Arthroplasty
The Journal of Arthroplasty Vol. 23 No. 6 2008
Kinematics of the Stiff Total Knee Arthroplasty Gavin C. Pereira, MBBS, FRCS (Tr & Orth),* Michael Walsh, PhD,* Bradley Wasserman, BS,* Scott Banks, PhD,y William L. Jaffe, MD,* and Paul E. Di Cesare, MD*
Abstract: The kinematics of 10 total knee replacements with poor flexion (b90°) were compared with 11 replacements with good flexion (N110°) at a mean of 3 years from surgery using optical calibration with implant shape-matching techniques from radiographs taken in standing, early-lunge, and late-lunge positions. There were no significant differences between groups in anteroposterior translation of the medial and lateral femoral condyles or tibial rotation during standing and early lunge. Groups differed in amount of posterior translation of the femoral condyles during late lunge because of the poor-flexion group's inability to achieve the same amount of flexion as the good-flexion group. Poor flexion after total knee arthroplasty, we conclude, is not associated with abnormal kinematics in the setting of well-aligned, well-fixed implants. Key words: total knee arthroplasty, stiffness, kinematics, fixed bearing, cruciate-retaining, poor flexion. © 2008 Elsevier Inc. All rights reserved.
commonly used terms such as rollback (posterior translation of femoral condyles), medial pivoting (medial condyle translating much less than lateral condyle), and the screw-home mechanism (external rotation of the tibia on the femur during flexion to full extension) [1,6-9]. These concepts have been applied to the study of kinematics of the total knee replacement, and extensive research has been published regarding the kinematics of cruciateretaining vs posterior-stabilized [10,11] and fixedbearing vs mobile-bearing [12,13] total knee replacements. Studies have also been performed on the kinematics of deep flexion [12,14] and kneeling [14,15] after total knee arthroplasty. All the aforementioned studies were performed on TKAs that demonstrated good or excellent flexion. Studies done on TKAs with poor kinematics, however, have shown that abnormal kinematics can lead to poor patient function [16,17] and early bearing surface wear [18-20]. Stiehl et al [21] showed that condylar geometry can drive poor kinematics and lead to increased wear. The incidence of stiffness after TKA is difficult to ascertain due to lack of a universally accepted definition. If postoperative flexion of less than 90°
A major postoperative goal after total knee arthroplasty (TKA) is to achieve flexion beyond 90°. Most activities that involve the knee require a range of motion (ROM) between 10° and 120° [1]. It is estimated that one requires 67° of flexion at the knee for effective walking, 85° of flexion to climb stairs, and 95° for rising from a chair comfortably [2]. Achieving less than 90° of flexion often leads to poor functionality and patient dissatisfaction [3]. Kinematics is the study of the motion of a joint without taking into account the forces acting upon it. The kinematics of the normal human knee has been studied using various in vivo and in vitro techniques [4-6]. These studies have given rise to
From the *Department of Orthopaedic Surgery, Musculoskeletal Research Center, NYU Hospital for Joint Diseases, New York, New York; and yDepartment of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida. Submitted September 25, 2006; accepted July 19, 2007. No benefits or funds were received in support of the study. Reprint requests: Paul E. Di Cesare, MD, Department of Orthopaedic Surgery, UC Davis Medical Center, Sacramento, CA 95817. © 2008 Elsevier Inc. All rights reserved. 0883-5403/08/2306-0016$34.00/0 doi:10.1016/j.arth.2007.07.015
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is used as a criterion, then recent studies indicate an incidence of 3.7% [22] to 6.3% [3]. A search of the literature found no kinematics study performed on stiff knees with limited range of motion. The hypothesis of this study was that the kinematics of knees with limited flexion differed significantly from those that have satisfactory flexion.
Materials and Methods This study was approved by the authors' institutional review board. The study population comprised 2 groups of patients: Those who had a maximum postoperative passive knee flexion of below 90° (classified as poor-flexion TKA) and those who had a maximum passive flexion of N110° (classified as good-flexion TKAs); the indication for surgery in all cases was osteoarthritis, and all procedures had been performed a minimum of 2 years previously. Patients with maximum flexion between 90° and 110° were excluded to avoid an overlap between clinical and radiological flexion. Criteria for inclusion included well-aligned implants and no evidence of loosening, wear, or infection. Clinical range of motion of the knee was measured using a goniometer. A deviation of N3° from the mechanical axis or posterior slope below 3° or above 7° were grounds for exclusion. The first 10 patients to fulfill the inclusion criteria in each group were chosen. All patients were operated on by 1 of 2 fellowship-trained senior adult reconstructive surgeons
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using a standard medial parapatellar approach. In all cases, the posterior cruciate ligament was either recessed or resected based on flexion-extension gap balancing. Varus-valgus deformities were corrected using appropriate soft tissue releases. Both surgeons used posterior referencing techniques for the femoral preparation and tibial posterior slope in all knees was set at 5°. The patella was resurfaced in all patients. In all instances, a cruciate-retaining knee design with a dished, metal-backed polyethylene tibial component (Stryker Scorpio Total Knee, Allendale, NJ). The prostheses were implanted using hand-mixed cement for the femoral, tibial, and patellar components (Simplex, Howmedica, Rutherford, NJ). All knees achieved above 110° of flexion at the end of implantation and above 90° of flexion after wound closure. All wounds were opened and closed with the knee in flexion. Anteroposterior (AP) and lateral radiographs of the knee were performed during standing, early lunge, and late lunge with the patient bearing as much weight on this knee as possible in a “closedchain” configuration. The standing radiographs were taken with the patient extending the knee to maximum extension. The early lunge was performed with the patient stepping up onto a step to achieve a comfortable flexion of about 70° to 80°. The late lunge was performed similarly, but the patient was asked to bend the knee to a maximum amount of tolerable flexion with the contralateral limb extended behind them to the maximum extent. Patients were allowed to hold
Fig. 1. Computerized 3D shape matching. A, Preoperative lateral radiograph. B, Postoperative lateral radiograph in late lunge. C, After matching a computer-generated model to radiograph (B), which allows 3D kinematics to be determined from the 2-dimensional image. Screws in views (B) and (C) are markers placed on the cassette used for the shapematching process.
896 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 Table 1. Study Population Demographics and Clinical and Radiographic Findings
Number Sex (M:F) Age Side (R:L) BMI Preoperative max flexion Max flexion at time of study Mean time from surgery Difference between post- and preoperative patellar thickness Difference between post- and preoperative posterior femoral condylar offset
Poor-flexion group (b90°)
Good-flexion group (N110°)
10 knees (9 patients) 3:6 66.8 (SD, 15.6) 4:6 29.4 (SD, 4.1) 105° (SD, 10°)
11 knees (8 patients) 3:5 57.2 (SD, 8.3) 6:5 33.7 (SD, 5.7) 106° (SD, 14°)
– .14 – .067 .81
83° (SD, 11°)
119° (SD, 9°)
b.0001
4.5 y
2.7 y
–
0 ± 3 mm
−2 ± 3 mm
.234
3 ± 5 mm
1 ± 3 mm
.182
P –
onto a support during the early and late lunges. The central x-ray beam was directed toward the knee with a marker on the film cassette. All radiographs were examined for alignment and loosening by an independent observer blinded to the degree of flexion. Posterior femoral offset and patellar thickness were measured from pre- and postoperative radiographs. The ratio of these measurements to the diameter of the femur on the lateral radiographs helped correct for magnification. Femoral and tibial components were examined for coronal, sagittal, and rotational alignment. Knee kinematics was determined using previously described methods [23]. Radiographs were
digitized, and the 3-dimensional (3D) position and orientation of the prosthesis were determined using model-based shape-matching techniques, manual matching, and image-space optimization routines (Fig. 1) [23]. The shape-matching technique— which allows one to determine 3D kinematics from 2-dimensional images—has previously been reported to have standard errors of approximately 1.5° of rotation and 2 mm of translation [24]. Flexion, varus-valgus, and axial rotations of the components to one another were mathematically calculated. The medial and lateral femoral condylar contact points to the tibial insert were estimated as the lowest point on each condyle relative to the tibial baseplate. Anteroposterior (sagittal) translations were measured from the tibial midcoronal line (TMCL). Anterior translation of the femur on the tibia was prefixed by a “+” (plus) sign, and posterior translation, by a “−” (minus) sign. Tibial rotation about a longitudinal axis was designated by a “+” (plus) sign for external rotation in relation to the femur, and a “−” (minus) sign for internal rotation. Hyperextension was designated by a “−” (minus) sign and flexion without a prefix. Two-sample t tests with equal variance were used to evaluate the data, with P b .05 set as level of significance.
Results Of the 10 patients selected in each group, 1 from the poor-flexion group and 2 from the good-flexion group were excluded because the imaging was of unsatisfactory quality to perform the shape-matching techniques. Hence, there were 9 patients (with 10 primary TKAs) in the poor-flexion TKA group and 8 patients (with 11 primary TKAs) who were in the good-flexion TKA group.
Table 2. Kinematic Determinants Mean flexion (degrees [SD]) (minus sign denotes hyperextesion) Standing Good-flexion group Poor-flexion group Early lunge Good-flexion group Poor-flexion group Late lunge Good-flexion group Poor-flexion group * P b .05.
Mean medial condyle position from TMCL (mm [SD]) (minus sign denotes posterior translation)
Mean lateral condyle position from TMCL (mm [SD]) (minus sign denotes posterior translation)
Tibial rotation from midsagittal line (degrees [SD]) (minus sign denotes internal rotation; plus sign denotes external rotation)
−1.0 (9.1) −1.8 (9.4)
−6.0 (2.3) −7.0 (3.0)
−6.8 (2.0) −7.0 (2.0)
−1.4 (2.9) +0.6 (3.4)
77.2 (18.9) 73.7 (12.7)
−3.5 (4.0) −3.8 (2.2)
−5.7 (2.1) −6.4 (2.6)
−4.3 (8.4) −5.7 (3.6)
106.0 (11.8) 83.6 * (22.5)
−8.9 (4.1) −5.5 * (2.1)
−11.6 (4.5) −8.2 * (3.2)
−5.8 (6.4) −5.9 (4.9)
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Table 4. Mean Translation of the Lateral Femoral Condyle
Standing to early lunge Early lunge to late lunge
Fig. 2. Medial condyle position with reference to the TMCL.
The mean follow-up was 3.4 years (range, 2.05.9 years) after the TKA. Demographics as well as clinical and radiographic findings of the poor-flexion (b90°) and good-flexion (N110°) groups are shown in Table 1. Radiographic assessment revealed that components were in satisfactory alignment. There were no radiological loose zones around the femoral or tibial components. Size match of the components was assessed to be compatible with bone geometry. There were no significant differences in posterior femoral offsets or of patellar thickness. Analysis of the kinematic determinants from shape matching revealed that the knees with good flexion had a mean radiological range of motion (ROM) of −1.0° (hyperextension) to 106.0° of flexion, while the poor-flexion knees flexed from −1.8° to 83.5° (Fig. 1, Table 2). Differences in our clinical and shape-matched ROM results are accounted for by the fact that anterior bowing of the femur and posterior slope of the tibial plateau cause arthroplasty components to have approximately 10° of hyperextension with neutral surgical alignment [25]. In the full weight-bearing standing position, the tibiofemoral contact point of the medial femoral condyle was seen to be positioned posterior to the TMCL at a mean distance of 6 ± 2.3 mm in the good-
Good-flexion group (mm)
Poor-flexion group (mm)
P
1±3
0.8 ± 3.7
.43
5.9 ± 4.1
2±3
.01
flexion group and 7 ± 3 mm in the poor-flexion group (Fig. 2). From this starting point, the medial condyle was seen to translate anteriorly by a mean of 2.6 mm ± 4 mm in the good-flexion group and 3.1 ± 3.2 mm in the poor-flexion group. There was no statistical difference in this anterior translation between the 2 groups (p = .37). In late lunge, the medial condyle translated posteriorly by a mean of 5.4 ± 3.4 mm from its position at early lunge in the good-flexion group and 1.6 ± 2.6 mm in the poorflexion group, a statistically significant difference (P b .005) (Table 3). In the standing position, the tibiofemoral contact point of the lateral femoral condyle was positioned posterior to the TMCL at a mean distance of 6.8 ± 2 mm in the good-flexion group and 7 ± 2 mm in the poor-flexion group (Table 4, Fig. 3). From this starting point, during early lunge, the lateral femoral condyle translated 1 ± 3 mm anteriorly in the goodflexion group and 0.8 ± 3.7 mm in the poor-flexion group, a difference that was not statistically significant (P = .43). In late lunge, the lateral femoral condyle translated posteriorly by a mean of 5.9 ± 4.1 mm from its position at early lunge in the goodflexion group and by a mean of 2 ± 3 mm in the poor-flexion group, a difference that was statistically significant (P = .01) (Table 4).
Table 3. Mean Translation of the Medial Femoral Condyle, mm Good-flexion group Poor-flexion group Standing to early lunge Early lunge to late lunge
P
2.6 ± 4
3.1 ± 3.2
.37
5.4 ± 3.4
1.6 ± 2.6
.005
Fig. 3. Lateral condyle positions with reference to the tibial midcoronal line.
898 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 Table 5. Mean Rotation of the Tibia
Standing to early lunge Early lunge to late lunge
Good-flexion group
Poor-flexion group
P
3° ± 7°
6° ± 4°
.13
1° ± 3°
1° ± 6°
.35
There was no statistical significance in tibial rotations between the 2 groups during standing, early lunge, or late lunge; all cases demonstrated a similar degree of internal rotation of the tibia, in relation to the femur, with increasing flexion (i.e., the screwhome mechanism was not significantly different in the 2 groups; see Table 5).
Discussion Causes of stiffness after TKA can be broadly classified as patient factors, technical factors, and postoperative complications (Table 6). In this study, patient factors were not significant; the group with good flexion was similar to the poor-flexion TKA group in terms of sex, age, preoperative range of motion, diagnoses, and body mass index (BMI) (the mean age of the patients with poor flexion was higher than the good-flexion group, but this did not reach statistical significance.) Neither did any technical factors appear to explain poor flexion. Patellofemoral thicknesses and posterior condylar offsets on preoperative and postoperative x-rays were found to be similar in the 2 groups (Table 1). No malpositions of the implants were seen in either the sagittal or coronal planes. The shape-matching process did not reveal any rotational malpositions either. Since the intraoperative flexion achieved was more than 90° in all cases, we believe that the other technical factors (eg, poor flexion/extension gaps and tight closure of the wound) were also precluded. None of the patients had postoperative complications (eg, infection, wound problems, hemarthrosis, heterotopic bone formation, or periprosthetic fractures.). We therefore undertook to examine if the stiffness could be explained solely on the basis of abnormal kinematics. Implants from the same manufacturer and similar operative procedures were used for the good- and poor-flexion groups; it would thus be expected that the kinematics would be similar provided all other patient and surgical variables were similar. This was borne out by the similarity in the pattern of kinematics in the standing and early lunge. Although the kinematics of the Scorpio Total Knee
has not been reported in detail, it has been part of larger studies [15,26]; moreover, many studies have reported on the kinematics of cruciate-retaining knees (results summarized in Table 3). With patients in the standing position, there was no significant difference in the position of the femoral condyles in relation to the TMCL between the poor-flexion and good-flexion groups (ie, they hyperextended to similar degrees), indicating that there was no difference in kinematics at full extension. These starting positions, when compared to other kinematics studies on cruciate-retaining knees, seem to be more posterior than some other TKA designs but similar to the Series 7000 TKA from the same manufacturer (Table 4). These locations are consistent with the deepest point on the sagittally curved tibial articular surface located 5 mm posterior to the TMCL. A more posterior starting point on the tibia has been shown to be advantageous for good flexion [16] as it prevents early impingement of the tibial component on the posterior femoral cortex (P Walker, personal communication). With patients in the early lunge position, mean flexion in the good-flexion group was 77.2° ± 18.9°, which was not significantly different from mean flexion for the poor-flexion group (73.7° ± 12.7°). During this arc of flexion, the medial femoral condyles in both groups translated anteriorly to a similar degree (P N .05). Similarly, the lateral femoral condyles in both groups translated anteriorly (P N .05). In both groups, the amount of anterior translation was more in the medial compartment than in the lateral compartment. Overall the medial femoral condyles of the replaced knees in both groups followed the pattern of the medial
Table 6. Factors Associated with Stiffness Following TKA [11,17,26,31-39] Patient-related factors
Surgical factors
Older age
Flexion-extension gap imbalance
Higher BMI
“Overstuffed” tibiofemoral or patellofemoral joint Posterior femoral/tibial osteophytes
Poor preoperative knee ROM Poor preoperative knee scores
Posterior condylar offset Prosthetic design selection Patella resurfacing Wound closure with knee in flexion Tibiofemoral angle
Postoperative factors Limited postoperative rehabilitation Infection Poor wound healing Hemarthrosis Heterotrophic ossification Periprosthetic fracture
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Table 7. Summary of Studies on the Kinematics of Cruciate-Retaining Total Knee Replacement Systems
Normal knee or implant type
Study Most et al [27]
NexGen CR (Zimmer, Warsaw, Ind.)
Bertin et al [28] NexGen (Zimmer, Warsaw, Ind.)
Li et al [29]
NexGen (Zimmer, Warsaw, Ind.)
Dennis et al [4] Various Banks et al [11] Series 7000 (Osteonics, Stryker, Mahwah, NJ)
femoral condyle of a normal knee, in a similar arc of flexion (ie, anterior translation). Johal [6], who examined tibiofemoral motion in normal knees using magnetic resonance imaging, found that, contrary to what might be expected, when the knee flexes from −5° of hyperextension to 90° of flexion, the medial femoral condyle translates anteriorly 2.2 ± 1.5 mm from the TMCL, that is, there was no “femoral rollback” until 90° flexion. The anterior translation of the lateral femoral condyles, on the other hand, would be considered abnormal in relation to normal knees; however, anterior translation with flexion is commonly observed in knee arthroplasty, and it is associated with the loss of the normal ligamentous and articular constraints. Most [27] observed that the anterior translations of both condyles occurred between 0° and 30°, but Bertin [28] noted that this anterior translation occurred between 60° and 80°; in a study by Li [29], it similarly occurred between 0° and 60° (Table 7). With patients in the late lunge position, there was a significant difference in the radiographic flexion achieved by the two groups (P b .05). While this difference reflects (deliberate) selection bias, it nevertheless confirms the clinical difference in ROM. The difference between the flexion achieved by the poor-flexion group in early and late lunge was not significant, nor was the difference between the early and late lunges in the good-flexion group. During maximum weight-bearing flexion, both femoral condyles in both groups translated posteriorly from their anterior translation that occurred during early lunge. The medial femoral condyle in
Distance of medial femoral Distance of lateral femoral condyle contact point condyle contact point from TMCL (mm [SD]) from TMCL (mm [SD]) Flexion angle (minus sign denotes (minus sign denotes (degrees) posterior position) posterior position) 0 30 150 0 20 40 60 80 100 0 60 90 0 90 0 107
−0.7 (±4.4) +1 (±6) −21.7 (±12.2) −4 (2.7) −5 (3) −5.9 (1.8) −6.3 (2.1) −5.9 (2) −7 (3.2) −3.9 (5.2) −2.4 (4.6) −3.4 (5.6) −2.2 (3.3) −2.3 (3.7) −7.5 (5.1) −1.4 (3.7)
−3.3 (±4.7) +0.4 (±4.8) 27.2 (±8.4) −3.4 (3.2) −6.5 (2) −6.4 (2) −6.7 (1.9) −7.2 (2.3) −7.8 (2.9) −2.6 (7.1) −5.7 (4.8) −8.2 (3.6) −5.7 (2.7) −7.4 (3.4) −6.9 (5.4) −3.0 (4.4)
the good-flexion group translated to a mean position of −8.9 ± 4.1 mm, whereas in the poor-flexion group it translated to a mean position of −5.5 ± 2.1 mm. Similarly, the lateral femoral condyles translated posteriorly to achieve mean distances from the TMCL of −11.6 ± 4.5 mm (good-flexion group) and −8.2 ± 3.2 mm (poor-flexion group). The femoral condyles of the poor-flexion knees did not achieve the same amount of posterior translation that the good-flexion knees did. We found no difference in tibial rotation between the 2 groups in either early or late lunge. As described by Johal [6], tibial internal rotation is linked with flexion and posterior translation in normal knees. This explains why the rotations might be similar in both groups in early lunge but not why they are similar in the late lunge, as the degree of flexion and the posterior translations of the lateral femoral condyles are different. Kanekasu et al [14] also found no relationship between the amount of tibial rotation and the maximum flexion achieved in a posterior cruciate-substituting arthroplasty; they suggested that knee rotation in such cases is more a function of foot and body position during the activity than of the absence of normal passive structures. The direction and pattern of rotation followed the kinematics of the normal knee, however, as well as that reported for cruciateretaining knees. Banks [11] reported an internal rotation of −6.5° ± 2.6° for cruciate-retaining knees in vivo, and Victor [10] reported a similar rotation of −6.3° ± 5.5° at full flexion. Li [30] noted a smaller tibial rotation of −3.4° from extension to 80° of flexion in healthy knees.
900 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 In conclusion, our in vivo study using radiographic 3D computerized matching techniques showed that the kinematics of the poor-flexion knee followed the pattern of the kinematics of the well-functioning control knees, albeit with a reduced range. This pattern was consistent with the pattern previously described for cruciateretaining knees but did not follow the pattern of normal knees. There was no difference in axial rotation of the tibia in the poor-flexion TKA group. Since the knee kinematics in both groups were similar until late lunge, it is unlikely that the lack of further posterior translation between early to late lunge in the poor flexion group was the cause the stiffness. Results from this study suggest that poor flexion after total knee arthroplasty is not associated with abnormal kinematics in the setting of well-aligned implants. The results of this study should add to the knowledge base of knee kinematics to further our understanding about TKA performance and hopefully help influence TKA design.
Acknowledgments The authors thank Sathappan S. Sathappan, MD, Charles Preston, MD, Samuel Park, MD, Daniel Ginat, MD, and Matthew Tericher, BS for their assistance in this project.
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Kinematics of the Stiff Total Knee Arthroplasty Gavin C. Pereira, MBBS, FRCS (Tr & Orth),* Michael Walsh, PhD,* Bradley Wasserman, BS,* Scott Banks, PhD,y William L. Jaffe, MD,* and Paul E. Di Cesare, MD*
Abstract: The kinematics of 10 total knee replacements with poor flexion (b90°) were compared with 11 replacements with good flexion (N110°) at a mean of 3 years from surgery using optical calibration with implant shape-matching techniques from radiographs taken in standing, early-lunge, and late-lunge positions. There were no significant differences between groups in anteroposterior translation of the medial and lateral femoral condyles or tibial rotation during standing and early lunge. Groups differed in amount of posterior translation of the femoral condyles during late lunge because of the poor-flexion group's inability to achieve the same amount of flexion as the good-flexion group. Poor flexion after total knee arthroplasty, we conclude, is not associated with abnormal kinematics in the setting of well-aligned, well-fixed implants. Key words: total knee arthroplasty, stiffness, kinematics, fixed bearing, cruciate-retaining, poor flexion. © 2008 Elsevier Inc. All rights reserved.
commonly used terms such as rollback (posterior translation of femoral condyles), medial pivoting (medial condyle translating much less than lateral condyle), and the screw-home mechanism (external rotation of the tibia on the femur during flexion to full extension) [1,6-9]. These concepts have been applied to the study of kinematics of the total knee replacement, and extensive research has been published regarding the kinematics of cruciateretaining vs posterior-stabilized [10,11] and fixedbearing vs mobile-bearing [12,13] total knee replacements. Studies have also been performed on the kinematics of deep flexion [12,14] and kneeling [14,15] after total knee arthroplasty. All the aforementioned studies were performed on TKAs that demonstrated good or excellent flexion. Studies done on TKAs with poor kinematics, however, have shown that abnormal kinematics can lead to poor patient function [16,17] and early bearing surface wear [18-20]. Stiehl et al [21] showed that condylar geometry can drive poor kinematics and lead to increased wear. The incidence of stiffness after TKA is difficult to ascertain due to lack of a universally accepted definition. If postoperative flexion of less than 90°
A major postoperative goal after total knee arthroplasty (TKA) is to achieve flexion beyond 90°. Most activities that involve the knee require a range of motion (ROM) between 10° and 120° [1]. It is estimated that one requires 67° of flexion at the knee for effective walking, 85° of flexion to climb stairs, and 95° for rising from a chair comfortably [2]. Achieving less than 90° of flexion often leads to poor functionality and patient dissatisfaction [3]. Kinematics is the study of the motion of a joint without taking into account the forces acting upon it. The kinematics of the normal human knee has been studied using various in vivo and in vitro techniques [4-6]. These studies have given rise to
From the *Department of Orthopaedic Surgery, Musculoskeletal Research Center, NYU Hospital for Joint Diseases, New York, New York; and yDepartment of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida. Submitted September 25, 2006; accepted July 19, 2007. No benefits or funds were received in support of the study. Reprint requests: Paul E. Di Cesare, MD, Department of Orthopaedic Surgery, UC Davis Medical Center, Sacramento, CA 95817. © 2008 Elsevier Inc. All rights reserved. 0883-5403/08/2306-0016$34.00/0 doi:10.1016/j.arth.2007.07.015
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is used as a criterion, then recent studies indicate an incidence of 3.7% [22] to 6.3% [3]. A search of the literature found no kinematics study performed on stiff knees with limited range of motion. The hypothesis of this study was that the kinematics of knees with limited flexion differed significantly from those that have satisfactory flexion.
Materials and Methods This study was approved by the authors' institutional review board. The study population comprised 2 groups of patients: Those who had a maximum postoperative passive knee flexion of below 90° (classified as poor-flexion TKA) and those who had a maximum passive flexion of N110° (classified as good-flexion TKAs); the indication for surgery in all cases was osteoarthritis, and all procedures had been performed a minimum of 2 years previously. Patients with maximum flexion between 90° and 110° were excluded to avoid an overlap between clinical and radiological flexion. Criteria for inclusion included well-aligned implants and no evidence of loosening, wear, or infection. Clinical range of motion of the knee was measured using a goniometer. A deviation of N3° from the mechanical axis or posterior slope below 3° or above 7° were grounds for exclusion. The first 10 patients to fulfill the inclusion criteria in each group were chosen. All patients were operated on by 1 of 2 fellowship-trained senior adult reconstructive surgeons
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using a standard medial parapatellar approach. In all cases, the posterior cruciate ligament was either recessed or resected based on flexion-extension gap balancing. Varus-valgus deformities were corrected using appropriate soft tissue releases. Both surgeons used posterior referencing techniques for the femoral preparation and tibial posterior slope in all knees was set at 5°. The patella was resurfaced in all patients. In all instances, a cruciate-retaining knee design with a dished, metal-backed polyethylene tibial component (Stryker Scorpio Total Knee, Allendale, NJ). The prostheses were implanted using hand-mixed cement for the femoral, tibial, and patellar components (Simplex, Howmedica, Rutherford, NJ). All knees achieved above 110° of flexion at the end of implantation and above 90° of flexion after wound closure. All wounds were opened and closed with the knee in flexion. Anteroposterior (AP) and lateral radiographs of the knee were performed during standing, early lunge, and late lunge with the patient bearing as much weight on this knee as possible in a “closedchain” configuration. The standing radiographs were taken with the patient extending the knee to maximum extension. The early lunge was performed with the patient stepping up onto a step to achieve a comfortable flexion of about 70° to 80°. The late lunge was performed similarly, but the patient was asked to bend the knee to a maximum amount of tolerable flexion with the contralateral limb extended behind them to the maximum extent. Patients were allowed to hold
Fig. 1. Computerized 3D shape matching. A, Preoperative lateral radiograph. B, Postoperative lateral radiograph in late lunge. C, After matching a computer-generated model to radiograph (B), which allows 3D kinematics to be determined from the 2-dimensional image. Screws in views (B) and (C) are markers placed on the cassette used for the shapematching process.
896 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 Table 1. Study Population Demographics and Clinical and Radiographic Findings
Number Sex (M:F) Age Side (R:L) BMI Preoperative max flexion Max flexion at time of study Mean time from surgery Difference between post- and preoperative patellar thickness Difference between post- and preoperative posterior femoral condylar offset
Poor-flexion group (b90°)
Good-flexion group (N110°)
10 knees (9 patients) 3:6 66.8 (SD, 15.6) 4:6 29.4 (SD, 4.1) 105° (SD, 10°)
11 knees (8 patients) 3:5 57.2 (SD, 8.3) 6:5 33.7 (SD, 5.7) 106° (SD, 14°)
– .14 – .067 .81
83° (SD, 11°)
119° (SD, 9°)
b.0001
4.5 y
2.7 y
–
0 ± 3 mm
−2 ± 3 mm
.234
3 ± 5 mm
1 ± 3 mm
.182
P –
onto a support during the early and late lunges. The central x-ray beam was directed toward the knee with a marker on the film cassette. All radiographs were examined for alignment and loosening by an independent observer blinded to the degree of flexion. Posterior femoral offset and patellar thickness were measured from pre- and postoperative radiographs. The ratio of these measurements to the diameter of the femur on the lateral radiographs helped correct for magnification. Femoral and tibial components were examined for coronal, sagittal, and rotational alignment. Knee kinematics was determined using previously described methods [23]. Radiographs were
digitized, and the 3-dimensional (3D) position and orientation of the prosthesis were determined using model-based shape-matching techniques, manual matching, and image-space optimization routines (Fig. 1) [23]. The shape-matching technique— which allows one to determine 3D kinematics from 2-dimensional images—has previously been reported to have standard errors of approximately 1.5° of rotation and 2 mm of translation [24]. Flexion, varus-valgus, and axial rotations of the components to one another were mathematically calculated. The medial and lateral femoral condylar contact points to the tibial insert were estimated as the lowest point on each condyle relative to the tibial baseplate. Anteroposterior (sagittal) translations were measured from the tibial midcoronal line (TMCL). Anterior translation of the femur on the tibia was prefixed by a “+” (plus) sign, and posterior translation, by a “−” (minus) sign. Tibial rotation about a longitudinal axis was designated by a “+” (plus) sign for external rotation in relation to the femur, and a “−” (minus) sign for internal rotation. Hyperextension was designated by a “−” (minus) sign and flexion without a prefix. Two-sample t tests with equal variance were used to evaluate the data, with P b .05 set as level of significance.
Results Of the 10 patients selected in each group, 1 from the poor-flexion group and 2 from the good-flexion group were excluded because the imaging was of unsatisfactory quality to perform the shape-matching techniques. Hence, there were 9 patients (with 10 primary TKAs) in the poor-flexion TKA group and 8 patients (with 11 primary TKAs) who were in the good-flexion TKA group.
Table 2. Kinematic Determinants Mean flexion (degrees [SD]) (minus sign denotes hyperextesion) Standing Good-flexion group Poor-flexion group Early lunge Good-flexion group Poor-flexion group Late lunge Good-flexion group Poor-flexion group * P b .05.
Mean medial condyle position from TMCL (mm [SD]) (minus sign denotes posterior translation)
Mean lateral condyle position from TMCL (mm [SD]) (minus sign denotes posterior translation)
Tibial rotation from midsagittal line (degrees [SD]) (minus sign denotes internal rotation; plus sign denotes external rotation)
−1.0 (9.1) −1.8 (9.4)
−6.0 (2.3) −7.0 (3.0)
−6.8 (2.0) −7.0 (2.0)
−1.4 (2.9) +0.6 (3.4)
77.2 (18.9) 73.7 (12.7)
−3.5 (4.0) −3.8 (2.2)
−5.7 (2.1) −6.4 (2.6)
−4.3 (8.4) −5.7 (3.6)
106.0 (11.8) 83.6 * (22.5)
−8.9 (4.1) −5.5 * (2.1)
−11.6 (4.5) −8.2 * (3.2)
−5.8 (6.4) −5.9 (4.9)
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Table 4. Mean Translation of the Lateral Femoral Condyle
Standing to early lunge Early lunge to late lunge
Fig. 2. Medial condyle position with reference to the TMCL.
The mean follow-up was 3.4 years (range, 2.05.9 years) after the TKA. Demographics as well as clinical and radiographic findings of the poor-flexion (b90°) and good-flexion (N110°) groups are shown in Table 1. Radiographic assessment revealed that components were in satisfactory alignment. There were no radiological loose zones around the femoral or tibial components. Size match of the components was assessed to be compatible with bone geometry. There were no significant differences in posterior femoral offsets or of patellar thickness. Analysis of the kinematic determinants from shape matching revealed that the knees with good flexion had a mean radiological range of motion (ROM) of −1.0° (hyperextension) to 106.0° of flexion, while the poor-flexion knees flexed from −1.8° to 83.5° (Fig. 1, Table 2). Differences in our clinical and shape-matched ROM results are accounted for by the fact that anterior bowing of the femur and posterior slope of the tibial plateau cause arthroplasty components to have approximately 10° of hyperextension with neutral surgical alignment [25]. In the full weight-bearing standing position, the tibiofemoral contact point of the medial femoral condyle was seen to be positioned posterior to the TMCL at a mean distance of 6 ± 2.3 mm in the good-
Good-flexion group (mm)
Poor-flexion group (mm)
P
1±3
0.8 ± 3.7
.43
5.9 ± 4.1
2±3
.01
flexion group and 7 ± 3 mm in the poor-flexion group (Fig. 2). From this starting point, the medial condyle was seen to translate anteriorly by a mean of 2.6 mm ± 4 mm in the good-flexion group and 3.1 ± 3.2 mm in the poor-flexion group. There was no statistical difference in this anterior translation between the 2 groups (p = .37). In late lunge, the medial condyle translated posteriorly by a mean of 5.4 ± 3.4 mm from its position at early lunge in the good-flexion group and 1.6 ± 2.6 mm in the poorflexion group, a statistically significant difference (P b .005) (Table 3). In the standing position, the tibiofemoral contact point of the lateral femoral condyle was positioned posterior to the TMCL at a mean distance of 6.8 ± 2 mm in the good-flexion group and 7 ± 2 mm in the poor-flexion group (Table 4, Fig. 3). From this starting point, during early lunge, the lateral femoral condyle translated 1 ± 3 mm anteriorly in the goodflexion group and 0.8 ± 3.7 mm in the poor-flexion group, a difference that was not statistically significant (P = .43). In late lunge, the lateral femoral condyle translated posteriorly by a mean of 5.9 ± 4.1 mm from its position at early lunge in the goodflexion group and by a mean of 2 ± 3 mm in the poor-flexion group, a difference that was statistically significant (P = .01) (Table 4).
Table 3. Mean Translation of the Medial Femoral Condyle, mm Good-flexion group Poor-flexion group Standing to early lunge Early lunge to late lunge
P
2.6 ± 4
3.1 ± 3.2
.37
5.4 ± 3.4
1.6 ± 2.6
.005
Fig. 3. Lateral condyle positions with reference to the tibial midcoronal line.
898 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 Table 5. Mean Rotation of the Tibia
Standing to early lunge Early lunge to late lunge
Good-flexion group
Poor-flexion group
P
3° ± 7°
6° ± 4°
.13
1° ± 3°
1° ± 6°
.35
There was no statistical significance in tibial rotations between the 2 groups during standing, early lunge, or late lunge; all cases demonstrated a similar degree of internal rotation of the tibia, in relation to the femur, with increasing flexion (i.e., the screwhome mechanism was not significantly different in the 2 groups; see Table 5).
Discussion Causes of stiffness after TKA can be broadly classified as patient factors, technical factors, and postoperative complications (Table 6). In this study, patient factors were not significant; the group with good flexion was similar to the poor-flexion TKA group in terms of sex, age, preoperative range of motion, diagnoses, and body mass index (BMI) (the mean age of the patients with poor flexion was higher than the good-flexion group, but this did not reach statistical significance.) Neither did any technical factors appear to explain poor flexion. Patellofemoral thicknesses and posterior condylar offsets on preoperative and postoperative x-rays were found to be similar in the 2 groups (Table 1). No malpositions of the implants were seen in either the sagittal or coronal planes. The shape-matching process did not reveal any rotational malpositions either. Since the intraoperative flexion achieved was more than 90° in all cases, we believe that the other technical factors (eg, poor flexion/extension gaps and tight closure of the wound) were also precluded. None of the patients had postoperative complications (eg, infection, wound problems, hemarthrosis, heterotopic bone formation, or periprosthetic fractures.). We therefore undertook to examine if the stiffness could be explained solely on the basis of abnormal kinematics. Implants from the same manufacturer and similar operative procedures were used for the good- and poor-flexion groups; it would thus be expected that the kinematics would be similar provided all other patient and surgical variables were similar. This was borne out by the similarity in the pattern of kinematics in the standing and early lunge. Although the kinematics of the Scorpio Total Knee
has not been reported in detail, it has been part of larger studies [15,26]; moreover, many studies have reported on the kinematics of cruciate-retaining knees (results summarized in Table 3). With patients in the standing position, there was no significant difference in the position of the femoral condyles in relation to the TMCL between the poor-flexion and good-flexion groups (ie, they hyperextended to similar degrees), indicating that there was no difference in kinematics at full extension. These starting positions, when compared to other kinematics studies on cruciate-retaining knees, seem to be more posterior than some other TKA designs but similar to the Series 7000 TKA from the same manufacturer (Table 4). These locations are consistent with the deepest point on the sagittally curved tibial articular surface located 5 mm posterior to the TMCL. A more posterior starting point on the tibia has been shown to be advantageous for good flexion [16] as it prevents early impingement of the tibial component on the posterior femoral cortex (P Walker, personal communication). With patients in the early lunge position, mean flexion in the good-flexion group was 77.2° ± 18.9°, which was not significantly different from mean flexion for the poor-flexion group (73.7° ± 12.7°). During this arc of flexion, the medial femoral condyles in both groups translated anteriorly to a similar degree (P N .05). Similarly, the lateral femoral condyles in both groups translated anteriorly (P N .05). In both groups, the amount of anterior translation was more in the medial compartment than in the lateral compartment. Overall the medial femoral condyles of the replaced knees in both groups followed the pattern of the medial
Table 6. Factors Associated with Stiffness Following TKA [11,17,26,31-39] Patient-related factors
Surgical factors
Older age
Flexion-extension gap imbalance
Higher BMI
“Overstuffed” tibiofemoral or patellofemoral joint Posterior femoral/tibial osteophytes
Poor preoperative knee ROM Poor preoperative knee scores
Posterior condylar offset Prosthetic design selection Patella resurfacing Wound closure with knee in flexion Tibiofemoral angle
Postoperative factors Limited postoperative rehabilitation Infection Poor wound healing Hemarthrosis Heterotrophic ossification Periprosthetic fracture
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Table 7. Summary of Studies on the Kinematics of Cruciate-Retaining Total Knee Replacement Systems
Normal knee or implant type
Study Most et al [27]
NexGen CR (Zimmer, Warsaw, Ind.)
Bertin et al [28] NexGen (Zimmer, Warsaw, Ind.)
Li et al [29]
NexGen (Zimmer, Warsaw, Ind.)
Dennis et al [4] Various Banks et al [11] Series 7000 (Osteonics, Stryker, Mahwah, NJ)
femoral condyle of a normal knee, in a similar arc of flexion (ie, anterior translation). Johal [6], who examined tibiofemoral motion in normal knees using magnetic resonance imaging, found that, contrary to what might be expected, when the knee flexes from −5° of hyperextension to 90° of flexion, the medial femoral condyle translates anteriorly 2.2 ± 1.5 mm from the TMCL, that is, there was no “femoral rollback” until 90° flexion. The anterior translation of the lateral femoral condyles, on the other hand, would be considered abnormal in relation to normal knees; however, anterior translation with flexion is commonly observed in knee arthroplasty, and it is associated with the loss of the normal ligamentous and articular constraints. Most [27] observed that the anterior translations of both condyles occurred between 0° and 30°, but Bertin [28] noted that this anterior translation occurred between 60° and 80°; in a study by Li [29], it similarly occurred between 0° and 60° (Table 7). With patients in the late lunge position, there was a significant difference in the radiographic flexion achieved by the two groups (P b .05). While this difference reflects (deliberate) selection bias, it nevertheless confirms the clinical difference in ROM. The difference between the flexion achieved by the poor-flexion group in early and late lunge was not significant, nor was the difference between the early and late lunges in the good-flexion group. During maximum weight-bearing flexion, both femoral condyles in both groups translated posteriorly from their anterior translation that occurred during early lunge. The medial femoral condyle in
Distance of medial femoral Distance of lateral femoral condyle contact point condyle contact point from TMCL (mm [SD]) from TMCL (mm [SD]) Flexion angle (minus sign denotes (minus sign denotes (degrees) posterior position) posterior position) 0 30 150 0 20 40 60 80 100 0 60 90 0 90 0 107
−0.7 (±4.4) +1 (±6) −21.7 (±12.2) −4 (2.7) −5 (3) −5.9 (1.8) −6.3 (2.1) −5.9 (2) −7 (3.2) −3.9 (5.2) −2.4 (4.6) −3.4 (5.6) −2.2 (3.3) −2.3 (3.7) −7.5 (5.1) −1.4 (3.7)
−3.3 (±4.7) +0.4 (±4.8) 27.2 (±8.4) −3.4 (3.2) −6.5 (2) −6.4 (2) −6.7 (1.9) −7.2 (2.3) −7.8 (2.9) −2.6 (7.1) −5.7 (4.8) −8.2 (3.6) −5.7 (2.7) −7.4 (3.4) −6.9 (5.4) −3.0 (4.4)
the good-flexion group translated to a mean position of −8.9 ± 4.1 mm, whereas in the poor-flexion group it translated to a mean position of −5.5 ± 2.1 mm. Similarly, the lateral femoral condyles translated posteriorly to achieve mean distances from the TMCL of −11.6 ± 4.5 mm (good-flexion group) and −8.2 ± 3.2 mm (poor-flexion group). The femoral condyles of the poor-flexion knees did not achieve the same amount of posterior translation that the good-flexion knees did. We found no difference in tibial rotation between the 2 groups in either early or late lunge. As described by Johal [6], tibial internal rotation is linked with flexion and posterior translation in normal knees. This explains why the rotations might be similar in both groups in early lunge but not why they are similar in the late lunge, as the degree of flexion and the posterior translations of the lateral femoral condyles are different. Kanekasu et al [14] also found no relationship between the amount of tibial rotation and the maximum flexion achieved in a posterior cruciate-substituting arthroplasty; they suggested that knee rotation in such cases is more a function of foot and body position during the activity than of the absence of normal passive structures. The direction and pattern of rotation followed the kinematics of the normal knee, however, as well as that reported for cruciateretaining knees. Banks [11] reported an internal rotation of −6.5° ± 2.6° for cruciate-retaining knees in vivo, and Victor [10] reported a similar rotation of −6.3° ± 5.5° at full flexion. Li [30] noted a smaller tibial rotation of −3.4° from extension to 80° of flexion in healthy knees.
900 The Journal of Arthroplasty Vol. 23 No. 6 September 2008 In conclusion, our in vivo study using radiographic 3D computerized matching techniques showed that the kinematics of the poor-flexion knee followed the pattern of the kinematics of the well-functioning control knees, albeit with a reduced range. This pattern was consistent with the pattern previously described for cruciateretaining knees but did not follow the pattern of normal knees. There was no difference in axial rotation of the tibia in the poor-flexion TKA group. Since the knee kinematics in both groups were similar until late lunge, it is unlikely that the lack of further posterior translation between early to late lunge in the poor flexion group was the cause the stiffness. Results from this study suggest that poor flexion after total knee arthroplasty is not associated with abnormal kinematics in the setting of well-aligned implants. The results of this study should add to the knowledge base of knee kinematics to further our understanding about TKA performance and hopefully help influence TKA design.
Acknowledgments The authors thank Sathappan S. Sathappan, MD, Charles Preston, MD, Samuel Park, MD, Daniel Ginat, MD, and Matthew Tericher, BS for their assistance in this project.
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