The goals of total knee arthroplasty (TKA) are relieving pain, improving knee motion, and allowing patients to return to activities of daily living. However, one major limitation of state-of-the-art TKA is failure to predictably achieve satisfying range of motion (ROM).1–4
Since the late 1990s, many implant manufacturers have modified traditional total knee designs to improve knee flexion.5–8However, the improvement in ROM with these so-called high-flexion knees is highly variable and is far from enough.
A multitude of factors make it difficult for TKA to produce high flexion, including posterior impingement, lack of enough rollback of the femoral condyles on the tibial plateau, the restraint of ligaments and capsules, and too much tibiofemoral congruence.9,10The extensor mechanism is also a major constraint preventing a replaced knee from bending deeply, although it maintains anteroposterior stability and powers knee extension.11 Being an essential component of the extensor mechanism, thepatella can increase the moment arm of the extensor mechanism and alter the magnitude and direction of the forces exerted by the extensor mechanism.12,13So the anterior part of modern high-flexion knee implants should be designed to improve the kinematics and biomechanics of extensor mechanism so as to accommodate high knee flexion after TKA.
Compared to traditional tibial insert, the high-flex insert of Genesis II PS knee system (Smith & Nephew, Memphis, TN, USA) is characterized with a chamfered anterior post and a chamfered anterior lip with the aim of optimizing the kinematics and biomechanics of extensor mechanism. In this study, we measured the patellar tracking and patellar ligament tension in high knee flexion after TKA using these two kinds of tibial inserts. We hypothesized that implant designs of eliminating impingement between extensor mechanism and implant in high knee flexion would alter patellar tracking and reduce patellar ligament tension, thus facilitating high knee flexion.
Preparation of cadaveric specimens
Six fresh-frozen human whole-lower-extremity cadaveric specimens (four right and two left) were used for testing. All knees were macroscopically intact and radiologically normal with no previous history of surgery and were capable of flexing from full extension to 150°of flexion. The specimens were stored at −20°C and thawed overnight at room temperature ((23±2)°C) before testing. Skin and subcutaneous tissue were removed, but the articular capsule, ligaments, tendons and quadriceps muscles were carefully preserved and identified. The femur remained intact, and the tibia was cut 25 cm from the medial joint line for mounting. The fibula was resected distal to the proximal tibiofibular joint. The specimens were kept moist with 0.9% normal saline solution to prevent drying during testing.
The femoral upper end and tibial lower end were mounted into the femoral and ankle pivoting mechanisms, respectively, of an Oxford-type testing rig (Figure 1).14 The two pivoting mechanisms simulated the hip and ankle joints and allowed unconstrained motion of the knee. The load was created by a weight, cable, and pulley system on the quadriceps muscles: 20 N for vastus medialis, 25 N for vastus lateralis, and 30 N for rectus femoris and vastus intermedius.15The direction of the loading force was approximately along the line of action of each muscle group. A handle was connected to the femoral pivoting mechanisms via a cable. Pulling the handle made the femoral pivoting mechanisms glide up and down along the vertical rod and drove the knee from full extension to 150°of flexion.
Procedures of TKA and measurement of patellar tracking and patellar ligament tension
A standard TKA was performed using the Genesis II PS knee system (Smith & Nephew, Memphis, TN, USA). There were two kinds of tibial inserts: the traditional and the high-flex (Figure 2). The design modifications of the high-flex insert with respect to the traditional insert included a chamfered anterior lip and a chamfered anterior post. The standard surgical protocol and instruments were used for component implantation. All surgeries were performed by one senior staff surgeon. Exposure was performed through a medial parapatellar arthrotomy. The surgical methods included posterior referencing and measured cutting techniques. The posterior cruciate ligament was removed. The patella remained intact without resurfacing. No soft-tissue release was required after implantation of the components. The arthrotomy and incision were carefully closed to avoid overtensioning of the soft tissues.
Patellar tracking relative to the femur was measured using an Optotrak Certus system (Northern Digital, Waterloo, Canada). The root-mean-square accuracy for each infrared-emitting diode (IRED) was 0.1 mm in the plane of flexion and 0.15 mm perpendicular to the plane of flexion. Marker arrays, each with four IREDs, were attached firmly to the patella, femur, and tibia using screws. Bony landmarks were then registered using the Optotrak pointer to relate the marker array coordinate system to an anatomic coordinate system. In the femoral coordinate system, the origin was defined as the midpoint of the medial and lateral epicondyles. The z axis passed through the medial and lateral epicondyles and pointed laterally. The x axis pointed posteriorly. The y axis therefore pointed distally. The position of the patella was defined as zero with the knee in full extension. The positive translation along the x and y axes represented posterior translation (TP) and distal translation (TD), respectively. The resultant translation (TR) in the sagittal plane was then calculated: TR = SQRT(TP2+TD2) (Figure 3).
The patellar ligament tension was measured directly using an S-type tension transducer (NK Biotechnical Engineering, Minneapolis, MN, USA). The transducer was clamped onto the patellar ligament mini-invasively without touching the inferior pole of the patella and tibial plateau from extension to 150°of flexion. The transducer was connected to a desktop computer. The system software dynamically displayed and recorded the data collected from the transducer.
For each specimen, the traditional tibial insert was used first. After the patellar tracking and patellar ligament tension were tested, the incision was opened. The femoral and tibial baseplate were not disturbed, and the high-flex insert was implanted while the traditional insert was removed. Then the incision was closed using the same suturing technique in the same suturing positions by the same surgeon. And the tests were repeated. Each measurement was repeated three times for each insert, and the mean value was calculated and compared for the two kinds of inserts.
Statistical analysis was performed using SPSS software (version 16.0; SPSS Inc., Chicago, IL, USA). The patellar resultant translation after TKA with the traditional tibial insert and with the high-flex tibial insert at 90°to 150°of flexion were compared using the paired-samples t-test. Patellar ligament tension after TKA with the traditional tibial insert and with the high-flex tibial insert were also compared using the paired-samples t-test. Pearson correlation coefficient (r) was used to determine the degree of association between the patellar resultant translation and the patellar ligament tension. A P value of <0.05 was considered statistically significant.
Patellar resultant translation
After TKA either with the traditional tibial insert or with the high-flex insert, a progressively distal and posterior translation of the patella was found as the knee flexion angle increased. However, the patella with the high-flex tibial insert translated less distally and more posteriorly compared with the patella with the traditional insert at each knee flexion angle (Figure 4). The patellar resultant translation with the high-flex insert was lower than that with the traditional insert and decreased by 6.83%, 6.35%, 5.96%, 5.64%, 5.96%, 6.46%, 6.13% at 90° to 150° of flexion angle (P= 0.046, 0.040, 0.041, 0.027, 0.012, 0.005, 0.006, respectively) (Table).
Patellar ligament tension
After TKA either with the traditional tibial insert or with the high-flex insert, patellar ligament tension progressively increased as the knee flexion angle increased. And the patellar ligament tension with the high-flex insert was lower than that with the traditional insert and decreased by 11.48%, 12.63%, 18.16%, 21.17%, 23.68%, 17.48%, 11.75% at 90° to 150° of flexion angle (P=0.062, 0.042, 0.064, 0.014, 0.002, 0.042, 0.201, respectively) (Table).
Correlation of patellar translation and ligament tension
Patellar resultant translation was highly correlated with patellar ligament tension (r=0.989; P=0.000).
Many factors influence ROM after TKA, including patient's preoperative activities, surgical techniques, design of the knee prostheses used, and postoperative rehabilitation.16Because the geometry and positioning of the knee prosthesis are different from those of the healthy knee, thus patellar tracking and extensor mechanism function is also altered after TKA.
In current study, we used Genesis II PS knee system as research implant. With this system, either a traditional tibial insert or a high-flex tibial insert can be used without changing tibial base plate and femoral component. There is impingement between patella and anterior post of the traditional insert, and between patellar ligament and anterior lip of the traditional insert in high knee flexion, which has been found in previous laboratory study. Then the so-called high-flex insert is produced with a chamfered anterior post designed to eliminate patellar impingement and a chamfered anterior lip with the aim of eliminating patellar ligament impingement in deep flexion. So, choosing these two kinds of inserts gave us the opportunity to investigate the impact of eliminating the impingement between extensor mechanism and implant on the kinematics and biomechanics of extensor mechanism in high knee flexion.
One cadaveric study reported that the patella showed a progressively posterior translation relative to the femur in the normal knee from full extension to 105°of flexion.17 A study using magnetic resonance imaging revealed that the patella slid into the intercondylar notch after flexion of >30°and translated posteriorly quickly and that the patella sank into the intercondylar notch deeply at 135°of flexion.18Our study revealed that the patella still shows a progressively posterior and distal translation relative to the femur as the knee flexion angle increases after TKA, just as earlier studies found. However, we found a difference in patellar tracking between two groups that differed only in the design of the tibial insert used. The patella with the high-flex insert showed a less distal and a more posterior translation than did the patella with the traditional insert. The TR showed a statistically significant decrease with the high-flex insert in high knee flexion. These findings indicated that high-flex insert eliminated the impingement between patella and anterior insert, reduced TR relative to the femur and made the patella sink more deeply into the femoral intercondylar notch in high knee flexion.
The changes of extensor mechanism kinematics would lead to the changes of tension within extensor mechanism. In this study, we found that in general, patellar ligament tension shows a progressive increase as knee flexion angle increases after TKA, with the highest patellar ligament tension being generated when the knee is in the highest flexion angle. Patellar ligament tension decreased after a high-flex insert was implanted at each knee flexion angle (100°, 120°, 130°, and 140°), a statistically significant change. These findings indicated that eliminating impingement between extensor mechanism and anterior insert reduced patellar ligament tension in high knee flexion. It was reasonable to assume that eliminating the extensor mechanism impingement and enhancing the deeper sinking of the patella into the intercondylar notch made the extensor mechanism relax relatively and thus reduced the tension of the extensor mechanism. The strong correlation between the TR and patellar ligament tension further supported our hypothesis. Our findings were consistent with a study in which the femoral prosthesis with nonanatomic trochlea design pushed the patella to translate anteriorly and distally and caused excessive strain on the anterior soft tissue of the knee.19
In current study, we found that the patellar ligament tension decreased by 21.17%, 23.68%, 17.48% at 120°, 130°, and 140° of knee flexion with statistically significant using the high-flex insert compared to the traditional insert. That's to say eliminating extensor mechanism impingement decreased the patellar ligament tension by about 1/5 in high knee flexion. Such a reduction in extensor mechanism tension may have some important clinical significance. As described previously, the extensor mechanism is a major constraint to knee flexion, so reduced tension would reduce the constraint on high knee flexion; in other words, reduced tension would facilitate high knee flexion after TKA. A study showed a strong statistical correlation between patellofemoral forces and quadriceps forces.20So, another clinical significance of our study was that reducing the tension of the extensor mechanism would reduce the patellofemoral forces and thus would decrease anterior knee pain and patellofemoral complications.
This study has several limitations. Firstly, this study is an in vitro cadaveric study. The testing rig used to drive knee motion did not completely simulate physiologic knee motion and the loading on the extensor mechanism did not completely replicate physiologic knee loading. However, the absolute biomechanics and kinematics of extensor mechanism were not the focus of our study. We concentrated on the changes in patellar tracking and patellar ligament tension due to the switch from traditional tibial insert to high-flex insert in the same specimen. Secondly, the two inserts came from one company. So we could not say that the problems found in Genesis II traditional insert were existed in other companies’ inserts or Genesis II high-flex tibial insert was better than other companies’ inserts. But the design modifications which had been proved to be able to improve the kinematics and biomechanics of extensor mechanism in our study could be mirrored by other companies. Lastly, this study only aimed to investigate the influence of two tibial insert on patellar ligament tension and patellar translations in the sagittal plane. We did not include other variables like patellar flexion angle and transverse displacement in our study design. Additional variables might be utilized to strongly enhance the conclusion.
Eliminating the impingement between extensor mechanism and implant in high knee flexion altered patellar tracking, enhanced the sinking of patella into femoral intercondylar notch and reduced patellar ligament tension, which would thus facilitate high knee flexion, although clinical studies will be required to determine the actual knee ROM and knee function after high-flexion TKA.
We are very grateful to Dr. Alfred J. Tria from the Departments of Pathology and Laboratory Medicine and Surgery, Robert Wood Johnson Medical School, and the Orthopaedic Center of New Jersey, for his helpful comments and criticisms on an earlier version of this manuscript that have resulted in significant improvements. We thank Katharine O'Moore-Klopf, ELS, for editorial assistance with the manuscript. We also thank XU Hai-jun, ZHANG Li-ang, DING Hui, ZHU Zhong-lin and DANG Xiao for technical assistance.
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