The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 8

Scientific Articles | August 01, 2004

Knee Kinematics with a High-Flexion Posterior Stabilized Total Knee Prosthesis: An in Vitro Robotic Experimental Investigation

Guoan Li, PhD¹; Ephrat Most, MS¹; Peter G. Sultan, MD¹; Steve Schule, MD¹; Shay Zayontz, MD¹; Sang Eun Park, MD¹; Harry E. Rubash, MD¹

¹ Bioengineering Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ 1215, Boston, MA 02114. E-mail address for G. Li: gli1@partners.org

View Disclosures and Other Information

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Zimmer, Inc. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Bioengineering Laboratory, Department of Orthopaedic Surgery, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 Aug 01;86(8):1721-1729

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Abstract

Background: An analysis of contemporary total knee arthroplasty reveals that, on the average, patients rarely flex the knee beyond 120°. The biomechanical mechanisms that inhibit further flexion after total knee arthroplasty are unknown. The objective of the present study was to investigate the capability of a single design of a fixed-bearing, high-flexion posterior stabilized total knee arthroplasty system (LPS-Flex) to restore the range of flexion to that of the intact knee.

Methods: Thirteen cadaveric human knees were tested, with use of a robotic testing system, before and after total knee arthroplasty with the LPS-Flex prosthesis. The passive path and the kinematics under an isolated quadriceps force of 400 N, under an isolated hamstring force of 200 N, and with these forces combined were determined. Posterior femoral translation of the lateral and medial femoral condyles and tibial rotation were recorded from 0° to 150° of flexion.

Results: The medial and lateral condyles of the intact knee translated posteriorly from full extension to 150°, reaching a mean peak (and standard deviation) of 22.9 ± 11.3 mm and 31.9 ± 12.5 mm, respectively, under the combined muscle forces. Following total knee arthroplasty, the amount of posterior femoral translation was lower than that observed in the intact knee. At 150°, approximately 90% of the intact posterior femoral translation was recovered by the total knee replacement. Internal tibial rotation was observed for all knees throughout the range of motion. The camspine mechanism engaged at approximately 80° and disengaged at 135°. Despite the absence of cam-spine engagement, further posterior femoral translation occurred from 135° to 150°.

Conclusions: The tibiofemoral articular geometry of the intact knee and the knee after total knee arthroplasty with use of the LPS-Flex design demonstrated similar kinematics at high flexion angles. The cam-spine mechanism enhanced posterior femoral translation only at the mid-range of flexion. The femoral component geometry of the LPS-Flex total knee prosthesis may improve posterior tibiofemoral articulation contact in high flexion angles.

Clinical Relevance: Although sufficient posterior femoral translation and tibial rotation were restored following total knee arthroplasty with the LPS-Flex prosthesis, these conditions alone may not be sufficient to fully produce the amount of knee flexion that is observed in the intact knee. The clinical outcome after total knee arthroplasty may be affected by other factors, such as preoperative range of motion, flexion space balancing, posterior tibiofemoral articular contact stability, quadriceps contraction, and patient motivation.

Figures in this Article

The human knee is capable of flexing up to approximately 160°^1-3. To squat and sit cross-legged for certain religious activities, an individual typically needs 111° to 165° of knee flexion¹; kneeling during prayer may demand flexion of >150°⁴, climbing up and down stairs as well as sitting on a chair requires 90° to 120° of flexion, and stepping into and out of a bathtub necessitates 135° of flexion⁵. High flexion of the knee is also essential for individuals who engage in such professions as construction and agriculture, as well as for individuals who participate in recreational activities such as gardening and golfing.

The range of flexion of the knee after total knee arthroplasty has been reported to be influenced by preoperative, intraoperative, and postoperative factors^6-15. However, clinical studies have rarely described knee flexion of >120°^6,16-29. Current prosthetic designs and surgical techniques may not be meeting the needs of patients who require knee flexion of >120° for their daily activities. In two recently published, multicenter, prospective clinical studies^17,30, average knee flexion following total knee arthroplasty was reported to be <120°. On the basis of the responses to a questionnaire completed by patients after total knee arthroplasty, Schai et al.³¹ found that fewer than half perceived that they could kneel easily, citing fear of harming the prosthesis as the main hindrance. In another study, squatting was possible for only thirty-one (45%) of sixty-eight knees after total knee arthroplasty despite the fact that the study was conducted on Asian patients who were accustomed to squatting³².

Using a computational model, Walker and Garg¹⁵ examined a variety of factors that may influence the range of motion of the knee after a cruciate-retaining total knee arthroplasty and showed that the posterior slope of the tibial component and the anterior-posterior positioning of the component may affect the range of flexion. In addition, recent work has shown that posterior femoral offset after cruciate-retaining total knee arthroplasty correlates with the range of flexion of the knee⁶. However, in general, posterior femoral translation has been referred to as a major factor affecting knee flexion^15,21,33. A reduction in posterior femoral translation has been shown to cause impingement of the posterior edge of the tibial component with the femoral shaft, thus limiting high flexion of the knee^15,34.

Most biomechanical studies related to knee arthroplasty have focused on knee function of <120° of flexion^15,35-37. As a result, the biomechanical mechanisms that limit higher knee flexion remain unclear. To our knowledge, no data have been reported on the capability of current total knee arthroplasty systems to reproduce posterior femoral translation of the knee of >120°.

In the present study, using an in vitro robotic testing system, we compared the posterior femoral translation and the tibial rotation of a single design of a high-flexion posterior stabilized total knee arthroplasty system with those in the intact knee in up to 150° of flexion. The objective of the study was to elucidate the intrinsic biomechanical mechanisms that affect the ability of the reconstructed knee to achieve flexion of >120°.

Materials and Methods

Materials and Methods | Results | Discussion | References

Thirteen fresh-frozen human cadaveric knee specimens were thawed overnight at room temperature prior to testing. The average age (and standard deviation) of the donors at the time of death was 67 ± 6 years (range, fifty-one to seventy-five years old). The femur and the tibia were cut approximately 25 cm from the joint line. The fibula was fixed to the tibia in its anatomical position with use of a cortical screw. The soft tissues around the knee joint were left intact. Both the tibia and the femur were potted in polymethylmethacrylate and secured in thick-walled aluminum cylinders. Before an experiment was conducted, the knee was preconditioned by flexing it manually between full extension and full flexion ten times. All knees tested were able to flex to 150°, the maximum flexion angle tested in this study.

A joint testing system with robotic technology was used to apply external loads to the knee specimens^36,38,39. The apparatus was composed of a robotic manipulator and a six-degrees-of-freedom load-cell, thus providing for both force and displacement control^36,38,39.

During an experiment, the femoral cylinder was mounted and rigidly fixed in a custom-designed clamp allowing for six-degrees-of-freedom positioning relative to the base of the manipulator. The tibial cylinder was mounted on the load-cell, which in turn was rigidly secured to the end-effector of the robotic manipulator. In this study, the specific knee kinematics addressed included the translations of the medial and lateral femoral condyles as well as internal-external tibial rotation. The centers of the medial and lateral condyles were defined as points on the transepicondylar line approximately 25 mm medial and lateral to the transepicondylar center^38-41. This value was obtained by measuring the dissected specimens after each test.

Simulation of the quadriceps and hamstring (semitendinosus-semimembranosus and biceps femoris) muscles and cocontraction of those muscles was performed with use of a pulley system^38,39. The tendons of each muscle were isolated and attached to hanging weights by means of tendon sutures and cables. Three muscle forces were applied as follows: (1) an isolated quadriceps force of 400 N, (2) a combined quadriceps and hamstring force (400 and 200 N, respectively), and (3) an isolated hamstring force of 200 N. These muscle forces were chosen to represent two extreme cases (an isolated quadriceps force and an isolated hamstring force) and one muscle-loading condition between the two. The specimen was sprayed regularly with 0.9% saline solution throughout the experiment.

After the knee was mounted onto the joint testing system, passive positions were defined at 1° increments between 0° and 150° of flexion. At each degree, knee positions for the remaining five degrees of freedom were determined such that the residual forces and moments at the knee joint center (the midpoint of the transepicondylar line) were minimized (<5 N and 0.5 N-m, respectively). These predetermined passive positions represented the relative position of the tibia with respect to the femur at which the joint carried a minimal load. This series of passive positions formed a passive path between 0° and 150°. The path was then used as the reference position for the subsequent application of simulated physiological loads during testing.

Following definition of the passive path, when the knee reached each of the selected flexion angles (0°, 30°, 60°, 90°, 120°, and 150°) along its passive path, a simulated muscle load was applied. The tibia was able to move along the remaining five degrees of freedom until reaching an equilibrium position where the applied load was balanced by the constraint forces generated inside the knee joint. The new knee position was then recorded by the robotic manipulator. The test was repeated at each selected flexion angle for all of the muscle loads.

After measurement of the intact knee kinematics in response to muscle loads, the same knee was then subjected to reconstruction by an orthopaedic surgeon using a posterior stabilized total knee arthroplasty system (NexGen LPS-Flex; Zimmer, Warsaw, Indiana). This total knee arthroplasty system facilitates tibiofemoral articulation at high flexion by the design modification of an increased thickness of the posterior wall of the femoral component by 2 mm compared with the standard NexGen LPS arthroplasty system. The purpose of this design modification is to extend the surface of the femoral component posteriorly to increase the articular contact area at high flexion angles (Fig. 1). A radiograph was made of each knee prior to testing, and component size was preliminarily determined on the basis of templating. The LPS-Flex total knee prosthesis was inserted through a standard medial parapatellar arthrotomy. Femoral cuts were performed first with the aid of an intramedullary guide to determine varus-valgus alignment, and the epicondylar axis was referenced for rotational alignment⁴². The tibial cut was then made with a posterior slope of 7° with use of an extramedullary system, with the tibial crest and the center of the tibial plateau used as reference points. The tibial component was aligned with the junction of the medial and middle thirds of the tibial tubercle. In all instances, the posterior cruciate ligament was resected. In this study, no soft-tissue release was performed in any of the specimens⁴³. The patella was not resurfaced. Trial components were inserted, and knee stability, passive range of motion, patellar tracking, and flexion-extension gaps were evaluated. Definitive sizes were determined intraoperatively. One specimen used a size-D component (58.6 and 64 mm in the anteroposterior and mediolateral directions, respectively), seven used a size-E component (62.5 and 68 mm, respectively), and five used a size-F component (66.5 and 72 mm, respectively). The polyethylene tibial component was 10-mm thick in seven knees, 12-mm thick in one, and 14-mm thick in five. The arthrotomy was closed with a continuous suture, as was the skin.

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Fig. 1: Improvement of tibiofemoral contact at high flexion of the knee in the high-flexion posterior stabilized total knee arthroplasty design is achieved by increasing the thickness of the posterior wall of the femoral component compared with that of the standard NexGen LPS total knee arthroplasty design.

The reconstructed knee was tested with use of the same protocol as described above for the intact knee. A passive path for the reconstructed knee was determined from 0° to 150° of flexion. The passive path served as a reference for subsequent application of external loads and for measurement of knee kinematics in response to the resultant forces. The muscle loads were again applied at 0°, 30°, 60°, 90°, 120°, and 150° of flexion, and the corresponding joint kinematics were measured by the robot. Radiographic images were made at selected flexion angles to ensure component stability.

After measuring the kinematics of the total knee arthroplasty, all soft tissues around the knee joint were dissected away to expose the components. The kinematics of the total knee arthroplasty was then reproduced on the specimen. The articulation of the total knee replacement was observed, and medial, lateral, and anteroposterior photographs were made. The cam-spine interaction during knee flexion was also observed.

Statistical Methods

A repeated-measures analysis of variance was used to detect statistical differences between the groups with respect to femoral translation and tibial rotation. A difference with a p value of <0.05 was considered significant.

Results

Materials and Methods | Results | Discussion | References

Posterior femoral translation and tibial rotation at the highest flexion (150°) along the passive path and under muscle loads are shown in Tables I and II. Because of the similar trend exhibited by the knees under the different loading conditions (passive, isolated quadriceps, combined hamstring-quadriceps, and isolated hamstring), only the kinematics of the knees under the combined muscle load throughout the entire range of flexion are presented.

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TABLE I Posterior Translation of the Lateral and Medial Femoral Condyles Under Four Loading Conditions^*

Passive Path†

Isolated Quadriceps Load (400 N)

Combined Quadriceps (400 N) and Hamstring (200 N) Load

Isolated Hamstring Load (200 N)

Lateral

Medial

Lateral

Medial

Lateral

Medial

Lateral

Medial

Posterior femoral translation at 150° of flexion (mm)

Intact knees

34.6 ± 10.4

24.7 ± 9.1

30.8 ± 14.5

20.3 ± 12.2

31.9 ± 12.5

22.9 ± 11.3

30.1 ± 14.7

20.9 ± 12.8

Knees after total knee arthroplasty

31.1 ± 9.7‡

20.7 ± 7.7‡

26.2 ± 13.5‡

18.5 ± 10.7

28.0 ± 12.2‡

20.4 ± 9.9

26.3 ± 13.6‡

19.3 ± 11.5

The values are given as the mean and the standard deviationNo external loadSignificant differences were detected between the knees after the LPS High-Flex total knee arthroplasty and the intact knees (p < 0.05)

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TABLE I Posterior Translation of the Lateral and Medial Femoral Condyles Under Four Loading Conditions^*

Passive Path†

Isolated Quadriceps Load (400 N)

Combined Quadriceps (400 N) and Hamstring (200 N) Load

Isolated Hamstring Load (200 N)

Lateral

Medial

Lateral

Medial

Lateral

Medial

Lateral

Medial

Posterior femoral translation at 150° of flexion (mm)

Intact knees

34.6 ± 10.4

24.7 ± 9.1

30.8 ± 14.5

20.3 ± 12.2

31.9 ± 12.5

22.9 ± 11.3

30.1 ± 14.7

20.9 ± 12.8

Knees after total knee arthroplasty

31.1 ± 9.7‡

20.7 ± 7.7‡

26.2 ± 13.5‡

18.5 ± 10.7

28.0 ± 12.2‡

20.4 ± 9.9

26.3 ± 13.6‡

19.3 ± 11.5

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TABLE II Internal Tibial Rotation at 150° of Flexion Under Four Loading Conditions^*

	Passive Path†	Isolated Quadriceps Load (400 N)	Combined Quadriceps (400 N) and Hamstring (200 N) Load	Isolated Hamstring Load (200 N)
Internal tibial rotation at 150° (deg)
Intact knees	11.5 ± 7.9	15.8 ± 8.0	10.4 ± 7.3	14.3 ± 6.7
Knees after total knee arthroplasty	12.2 ± 8.9	11.7 ± 10.2	8.8 ± 8.3	10.7 ± 9.5

The values are given as the mean and the standard deviationNo external load

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TABLE II Internal Tibial Rotation at 150° of Flexion Under Four Loading Conditions^*

	Passive Path†	Isolated Quadriceps Load (400 N)	Combined Quadriceps (400 N) and Hamstring (200 N) Load	Isolated Hamstring Load (200 N)
Internal tibial rotation at 150° (deg)
Intact knees	11.5 ± 7.9	15.8 ± 8.0	10.4 ± 7.3	14.3 ± 6.7
Knees after total knee arthroplasty	12.2 ± 8.9	11.7 ± 10.2	8.8 ± 8.3	10.7 ± 9.5

The values are given as the mean and the standard deviationNo external load

Posterior Femoral Translation and Tibial Rotation at Flexion Angles of =90°

Posterior translation of both the medial and lateral femoral condyles was found to increase with an increasing flexion angle before and after the posterior stabilized total knee arthroplasty (Fig. 2). In the intact knees, the posterior translation of the lateral femoral condyle was a mean (and standard deviation) of 3.2 ± 1.6 mm at 0° (full extension) and increased under muscle loads to 90° of flexion, reaching a mean value of 13.8 ± 7.0 mm at 90° (Fig. 2, A). After the total knee arthroplasty, the posterior translation of the lateral condyle was a mean of 4.3 ± 5.7 mm at 0° and increased to 6.7 ± 6.2 mm at 90°, which was significantly (p = 0.002) lower than the mean of 13.8 ± 7.0 mm observed in the intact knees at 90°.

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Fig. 2: Graphs comparing the posterior femoral translation of the intact knees and the knees after the high-flexion posterior stabilized total knee arthroplasty (TKA) under a combined quadriceps and hamstring load of 400 and 200 N, respectively, for (a) the lateral femoral condyle and (b) the medial femoral condyle. The asterisk indicates a significant difference (p < 0.05).

On the medial side, in the intact knees, the femoral condyle was posteriorly translated to a mean of 0.3 ± 1.7 mm at full extension under the combined muscle load and femoral translation increased to a mean of 9.1 ± 6.8 mm at 90° of flexion (Fig. 2, B). After total knee arthroplasty, there was no significant change in the amount of posterior translation of the medial condyle between 0° and 90° under the combined muscle load. At 90°, the mean posterior translation of the medial condyle was 2.6 ± 5.3 mm, which was significantly (p < 0.001) lower than the 9.1 ± 6.8 mm observed in the intact knees at 90°.

Internal rotation was significantly greater at 30° and 60° of flexion under combined muscle-loading conditions for the intact knees compared with that after total knee arthroplasty (Figs. 3 and 4). The reconstructed knees exhibited less internal tibial rotation than did the intact knees throughout the entire range of knee flexion except at full extension (Fig. 4).

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Fig. 3: Illustration of the posterior translation of the medial and femoral condyles under a combined quadriceps and hamstring load of 400 and 200 N, respectively, for (a) the intact knees and (b) the knees after the high-flexion posterior stabilized total knee arthroplasty (TKA). Note that greater posterior translation of the lateral femoral condyle indicates internal tibial rotation.

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Fig. 4: Internal tibial rotation of the intact knees and the knees after the total knee arthroplasty (TKA) under a combined quadriceps and hamstring load of 400 and 200 N, respectively. The asterisk indicates a significant difference (p < 0.05).

Posterior Femoral Translation and Tibial Rotation at >90° of Flexion

Between 90° and 150°, the lateral femoral condyle of the intact knees exhibited a further increase in posterior translation under the combined muscle load (Fig. 2). In the intact knees, at 150° of flexion, the lateral femoral condyle translated posteriorly a mean of 34.6 ± 10.4 mm on the passive path and 31.9 ± 12.5 mm under the muscle-loading conditions (Table I and Fig. 2, A). After the total knee arthroplasty, posterior translation of the lateral femoral condyle was similar to that of the intact knees both along the passive path and under muscle loads. The lateral condyle translated posteriorly a mean of 31.1 ± 9.7 mm on the passive path (approximately 90% of the value exhibited in the intact knee; Table I) and 28.0 ± 12.2 mm under combined muscle-loading (approximately 88% of the value exhibited in the intact knee; Table I and Fig. 2, A) at 150° of flexion. This amount of posterior femoral translation exhibited by the reconstructed knee was significantly less (p = 0.01 for the passive path and p = 0.013 under combined muscle load) than that observed for the intact knees.

With regard to the medial femoral condyle in the intact knees, posterior translation occurred on the passive path (mean, 24.7 ± 9.1 mm) and under muscle loads (mean, 22.9 ± 11.3 mm) at 150° of flexion (Table I). The reconstructed knees exhibited similar degrees of posterior femoral translation in comparison with the intact knee. The reconstructed knees exhibited a mean of 20.7 ± 7.7 mm of posterior translation without muscle loads (the passive path) and a mean of 20.4 ± 9.9 mm of posterior femoral translation under the combined muscle load. The values for the reconstructed knees on the passive path were significantly different from the corresponding values for the intact knees (p = 0.008), but the values under muscle-loading were not (p = 0.067).

Additional internal rotation occurred in both the intact knees and the reconstructed knees for flexion angles of >90° (Table II and Fig. 4). Tibial rotation was similar during passive flexion and under muscle-loading at these flexion angles. At 150° of flexion, under the combined muscle load, internal tibial rotation of the intact knee was a mean of 10.4° ± 7.3°, while the reconstructed knee exhibited a mean of 8.8° ± 8.3° of internal tibial rotation. No significant differences (p = 0.08) were found in terms of tibial rotation between the intact knee and the reconstructed knee at these flexion angles.

Visual Observation of Tibiofemoral Articulation

Beyond 135°, the tibiofemoral articular contact was observed to progress toward the posterior edge of the tibial polyethylene component under all of the applied loads. Under all loading conditions at 150°, the femoral component was observed to contact close to the posterior edge of the polyethylene component. Cam-spine engagement occurred between 70° and 80° of flexion. This engagement continued up to approximately 135°. Thereafter, cam-spine disengagement was observed, further posterior femoral translation was recorded, and posterior contact of the thigh and calf was noticed.

Discussion

Materials and Methods | Results | Discussion | References

The range of motion after total knee arthroplasty is an important factor with respect to the patient's overall functional outcome⁴⁴. However, following total knee arthroplasty, patients can rarely flex the knees beyond 120°^6,17,21,34. Although the mechanisms that hinder more flexion are unclear, the ability to restore posterior femoral translation after total knee arthroplasty has been shown to be an important factor in enhancing knee flexion^15,21,33,34. In the present study, after the high-flexion total knee arthroplasty, the knees generally exhibited less posterior femoral translation relative to that recorded for the intact knees, at flexion angles of up to 90° except at 0° (full extension) (Fig. 2). At these lower flexion angles, the amount of posterior femoral translation that occurs is determined, in part, by prosthetic geometry, the interaction between the sagittal curvature of the polyethylene tibial insert and the femoral component, and by the cam-spine interaction. For example, the posterior lip of the tibial component can apply an anteriorly directed force on the femoral component, thus functionally resisting further posterior femoral translation at lower flexion angles. However, after 90° of flexion, the reconstructed knees exhibited a dramatic increase in the magnitude of posterior femoral translation. Ultimately, approximately 90% of the amount of posterior translation that occurred in the intact knees was observed to occur in the reconstructed knees at 150° of flexion.

In the present study, the amount of posterior femoral translation of both the medial and lateral femoral condyles increased with the increasing flexion angle for the intact knees. In high flexion, the intact knee was highly constrained in response to external loads, indicating a tight flexion space between the tibia and the femur. Internal tibial rotation occurred throughout the range of flexion, and this corresponded to greater posterior translation of the lateral femoral condyle relative to the medial condyle. This result is consistent with those in other studies^3,4,45 as well as with our prior findings in a kinematic study of the knee at high flexion⁴⁰. In our previous study, we additionally reported that the forces in both the anterior cruciate ligament and posterior cruciate ligament were found to be minimal at 150° of flexion under various muscle-loading conditions⁴⁶. Those prior findings suggested that, in addition to the contribution of the cruciate ligaments, the compression of the menisci, posterior capsule, and other posterior soft-tissues may play an important role in enhancing posterior femoral translation during knee motion by applying an anteriorly directed force on the tibia at higher degrees of flexion. In addition, these structures may serve to enhance knee stability as the knee flexes to higher angles under muscle loads⁴⁶.

The cam-spine mechanism of the posterior stabilized total knee arthroplasty system examined in these experiments was designed to engage at approximately 80° of flexion³⁶. Function of the cam-spine mechanism was confirmed by our kinematics data. Due to engagement of the cam-spine mechanism, the rate of posterior femoral translation began to increase at approximately 90° of flexion (Fig. 2).

In the design of the high-flexion posterior stabilized total knee prosthesis, the cam-spine mechanism is intended to facilitate posterior femoral translation⁴⁷, thus improving knee flexion. In the present study, the amount of posterior femoral translation at high flexion in the reconstructed knee was comparable with that in the intact knee. However, the results of this study indicate that the cam-spine interaction may affect posterior femoral translation only at flexion angles between 90° and 135°. Beyond this range of flexion, cam-spine engagement was not observed consistently. Despite the absence of engagement, additional posterior femoral translation was found to occur as the knee flexed to higher angles. We speculate that compression of the posterior soft-tissue structures (such as the posterior capsule, pericapsular fat, muscle, and skin) may act to push the tibia anteriorly, resulting in continual posterior femoral translation of the total knee replacement at higher flexion angles. Additionally, since total knee arthroplasty involves alteration of the articular surface geometry of the intact knee as well as posterior cruciate ligament removal, the similar in vitro kinematics observed in the intact and the reconstructed knee at high flexion suggests that the posterior soft-tissues may play a similar and important role in both conditions to guide knee motion.

Clinical studies have rarely described knee flexion of >120° after total knee arthroplasty^{6,17,21,34,48}. Anouchi et al.¹⁷ examined various preoperative conditions in patients managed with a single design of a posterior stabilized total knee replacement (Advantim; Wright Medical Technology, Arlington, Tennessee) and concluded that all patients tend to flex the knee within a predictable range with a mean value of 107°. In an in vivo investigation of deep knee-bending, with use of fluoroscopic technology, Dennis et al.^30,49 found that the posterior stabilized total knee arthroplasty system (Press-Fit Condylar; Johnson and Johnson Professional, Raynham, Massachusetts) rolls back posteriorly during flexion, in a manner similar to that of intact knees. In both studies, tibiofemoral contact was observed at the posterior portion of the tibial component between 90° and 120° of flexion. These in vivo investigations also indicated that posterior femoral translation of the knee after reconstruction with a posterior stabilized total knee prosthesis can occur even at >90° of flexion. On the basis of both in vivo and in vitro observations, we believe that the capability of a total knee arthroplasty prosthesis to reproduce posterior femoral translation is a necessary condition for high knee flexion. However, the posterior femoral translation may not be sufficient to achieve higher knee flexion in patients. Additional factors are likely to play an important role in permitting the reconstructed knee to achieve higher flexion.

In our previous study⁴⁰ and in investigations done by others⁵⁰, it has been observed that, in the intact knee at high flexion angles, the femoral condyles roll off the tibial plateau posteriorly. This phenomenon has been shown to occur in a more pronounced fashion on the lateral side of the knee than on the medial side (Fig. 5). Posterior motion of the menisci may serve to effectively elongate the tibial plateau in the posterior direction, thus providing the intact knee with a dynamically extended articular surface. This particular function of the menisci with regard to the tibiofemoral articulation may be an important intrinsic mechanism that is present in the intact knee at high flexion angles.

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Fig. 5: T2-weighted magnetic resonance images, made in the sagittal plane, with the knee at 150° of flexion on its passive path. The images show the menisci compressed between the femur and the tibia in both the medial (a) and lateral (b) compartments at high flexion.

Following total knee arthroplasty, meniscal function is lost because of surgical resection of the structure. In a typical total knee arthroplasty system, the anterior-posterior dimensions of the tibial articular surface on the medial and lateral sides of the intact knee are greater than the comparable dimensions of the tibial polyethylene. As a consequence, in the standard total knee arthroplasty system, posterior femoral translation results in contact at the posterior portion of the reconstructed tibial articular surface. This might result in edge-loading of the posterior tibiofemoral joint. To mitigate this possibility, the tibial component should provide for enough posterior coverage to effectively elongate the articular surface and reduce the potential for edge-loading, thus improving knee stability at high flexion. One solution, as seen with the high flexion design examined in this study, is elongation of the femoral condyles to provide a greater contact area between the femoral and tibial components during high flexion. A potential surgical solution to improve posterior tibiofemoral articular contact in high flexion is to place the tibial component more posteriorly on the plateau (particularly with regard to the lateral side), which may enhance posterior coverage. Another consideration would be to extend the tibial component posteriorly, particularly on the lateral side, to accommodate for femoral rollback. The potential interaction between a posteriorly placed or, potentially, a posteriorly extended tibial component and posterior soft-tissue compression requires further investigation.

The present study describes the posterior translation of the femoral condyle centers, which were defined along the transepicondylar line of the femur^41,42. We chose to use the transepicondylar line as the flexion axis because of its relation to apparent osseous landmarks that make it reproducible on different knee specimens. The transepicondylar line on the medial side was defined as the center of the medial prominence of the medial epicondyle. By this definition, the medial femoral condylar translation will be larger than that described with use of the contact points^30,40,51,52. This may help to explain the variations in knee kinematics reported in the literature^30,40,51,52.

One limitation of this study is that the muscle loads simulated were lower than those estimated during physiological activities⁵³. This was due to the load capacity of the testing system. However, the present study allowed for comparison of the function of the NexGen LPS-Flex total knee arthroplasty system with that of the intact knee under the same loading conditions. Also, this study examined only a single design of a high-flexion total knee replacement and extrapolation of these results to other high-flexion knee designs should be done with caution.

We conclude that, although sufficient posterior femoral translation and tibial rotation were restored following arthroplasty with use of the LPS-Flex total knee replacement, these conditions alone may not be sufficient to fully produce the amount of knee flexion that is observed in the intact knee. Other factors such as preoperative range of motion, flexion space balancing, posterior tibiofemoral articular contact stability, quadriceps contraction, and patient motivation may affect the clinical results and the range of motion after total knee arthroplasty.

References

Materials and Methods | Results | Discussion | References

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Topics

femur ; knee region ; knee joint ; tibia ; knee replacement arthroplasty ; kinematics ; quadriceps ; knee flexion ; knee prosthesis, total

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Guoan Li, Ephrat Most, Peter G. Sultan, Steve Schule, Shay Zayontz, Sang Eun Park, Harry E. Rubash; Knee Kinematics with a High-Flexion Posterior Stabilized Total Knee Prosthesis: An in Vitro Robotic Experimental Investigation. The Journal of Bone & Joint Surgery. 2004 Aug;86(8):1721-1729.

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