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Effect of PCL on Flexion-Extension Gaps and Femoral Component Decision in TKA

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Abstract

This study was done to evaluate the change of medial-lateral gap in flexion and extension after posterior cruciate ligament (PCL) release in severely deformed knees and to determine how PCL release affects bone resection, rotation, and size of the femoral component and polyethylene thickness in converting to a PCL-sacrificed design. Thirty primary osteoarthritis patients with severe varus deformity or flexion contracture were enrolled. After releasing the PCL, the medial gap in extension increased by 1.2 mm, the lateral gap in extension increased by 0.3 mm, the medial gap in flexion increased by 4.5 mm, and the lateral gap in flexion increased by 3.4 mm. Compared with PCL-retained prostheses, the mean external rotation of the femoral component decreased by 1.6° in the PCL-sacrificed type. Polyethylene thickness increased by 1.2 mm. In 12 cases, a larger femoral component was needed. In 8 knees, the size of the femoral component and the thickness of polyethylene did not change; however, posterior femoral resection could be decreased. In 8 knees, thicker polyethylene was planned with slightly increased distal femoral resection. After PCL cutting, flexion gap increased significantly compared with extension gap; however, correction of varus deformity was not significant. Conversion to PCL-sacrificed design resulted in a decrease in external rotation of the femoral component and increased the size of the femoral component or the thickness of the polyethylene insert.

The posterior collateral ligament (PCL) acts as a central stabilizer to absorb force and prevent posterior subluxation. The PCL has an important role in making the femoral condyle roll back on the tibial plateau during flexion, thereby allowing the posterior condyles to clear the posterior aspect of the tibia in the high degrees of flexion and improving the mechanical efficiency of the extensor mechanism. In selecting the designs of total knee arthroplasty (TKA), whether the PCL should be retained or resected has been debated for many years, and several studies have been performed regarding the gap difference after PCL resection.^1-5

However, most study designs excluded knees with severe varus/valgus and flexion deformities, which are more favorable to PCL-sacrificed design. In general in these severely deformed knees, PCL-sacrificed design was used because PCL resection allows for extensive exposure and easy correction of angular deformities. Most studies provided only flexion-extension gaps, not medial-lateral gaps at flexion and extension. Furthermore, there is no study describing the change of the femoral component size and rotation or of the polyethylene insert after PCL cutting.

The purpose of this study was to evaluate the change in medial-lateral gaps on flexion and extension between PCL-retained and PCL-sacrificed TKAs in severely deformed knees, and to understand how PCL sacrificing affects the bone resection level, rotation of the femoral component, femoral component size, and polyethylene thickness in TKAs.

Materials and Methods

Thirty primary osteoarthritis patients (29 women; 1 man) were enrolled. Preoperative varus deformity was >20° or flexion contracture was >15°. All varus deformities were not corrected to neutral position on the valgus stress knee anteroposterior (AP) radiograph. Posterior cruciate ligament was intact in all cases as confirmed intraoperatively. Patient preoperative deformities are summarized in Table 1.

OrthoPilot (B. Braun Aesculap, Tuttlingen, Germany) navigation was used and an E.motion (B. Braun Aesculap) implant was used for preoperative planning.

Following a midline skin incision, we performed medial parapatellar arthrotomy with reduced incision of the quadriceps tendon (2 cm above the patella). After fixing transmitters on the femur and tibia, anatomic and kinematic values were registered. The anterior cruciate ligament and meniscus were resected, and stepwise medial release was done to correct the varus deformity. A tibial cut was made perpendicular to the mechanical axis with 0° of posterior slope under navigation control. The tibial cutting level was 9 mm off from the lateral plateau and the PCL was preserved along with its bony island. Medial-lateral gaps in full extension and 90° flexion were measured using a tensioning device with a distraction force (200 Nm) before femoral resection. The tensor consists of 2 upper plates, 1 lower platform plate, and the extraarticular main body. The device allows surgeons to measure the ligament balance and the medial-lateral gap while applying a constant distraction force. Femoral component planning was done using navigation, which provided information regarding amount of bone resection of the distal and posterior femur, femoral component size, femoral component rotation, and polyethylene thickness. After completing planning for the cruciate retaining type of femoral prosthesis, the PCL was cut and the medial-lateral gaps were reassessed in full extension and 90° flexion. Replanning for the femoral component was then done (Figure).

Figure: Navigational planning procedure. Posterior cruciate ligament (PCL) preserved case (A). Before PCL cutting, the medial and lateral gaps in extension are 7 and 10 mm, respectively. The medial and lateral gaps in 90° flexion are 5 and 8 mm, respectively. Size 4 femoral component is chosen with an external rotation of 6° and a 10-mm-thick polyethylene insert. Posterior cruciate ligament cutting case (B). After PCL cutting, the medial and lateral gap in flexion increased to 11 and 12 mm, respectively. External rotation of the femoral component decreased to 2° with a femoral component of the same size and a 12-mm-thick polyethylene insert thickness with slight increase of distal femoral cutting.

We compared the change of medial-lateral gaps on flexion and extension between PCL-retained and PCL-sacrificed state in these severely deformed knees. We also compared the femoral bone resection level, rotation of femoral component, femoral component size, and polyethylene thickness before and after PCL resection. Final implantation was done using E.motion ultra-congruency implants using polyethylene inserts with a more elevated anterior and posterior lip (B. Braun Aesculap).

Results

After cutting the PCL, the medial-lateral gaps in extension increased by 1.2 and 0.3 mm, respectively. There was a 0.9-mm difference between medial gap and lateral gap in extension. Medial gap in 90° flexion increased by 4.5 mm and lateral gap in 90° flexion increased by 3.4 mm. There was a 1.1-mm difference between medial gap and lateral gap in 90° flexion (Table 2). Regarding femoral component planning, after PCL cutting, the mean external rotation of the femoral component decreased by 1.6° (from a mean of 4.8° to a mean of 3.2°), and polyethylene insert thickness increased by a mean 1.2 mm. In 12 cases (40%), the femoral component size increased by 1 size to achieve well-balanced flexion-extension gaps after PCL cutting. In 9 of the 12 cases, the thickness of polyethylene insert did not change.

In 8 cases (26.7%), the size of the femoral component and the thickness of the polyethylene insert did not change, and the amount of posterior femoral resection decreased. In 8 cases (26.7%), the size of the femoral component did not change; however, thicker polyethylene was planned with slightly increased distal femoral cutting (Table 3). After PCL cutting, the mean posterior femoral bone resection amount decreased to 11.4 mm on the medial side and 8.3 mm on the lateral side, which was more similar to the implant thickness, compared with 14 mm on the medial side and 9.5 mm on the lateral side in cases of preserved PCL. However, the mean distal femoral resection was not significantly different before or after PCL resection (Table 4).

Discussion

To achieve stable tibiofemoral and patellofemoral joints in TKA, there must be accurate alignment of the joint components and balancing of the soft tissues using appropriate surgical techniques and well-designed implants.^2,6-8 After introduction of the PCL-sacrificed technique of TKA by Freeman, the superiority of either PCL-retained or PCL-sacrificed TKA has remained a source of controversy regarding knee kinematics, range of motion, osteolysis, polyethylene wear, and proprioception of the knee joint.^9-11 Proponents of the PCL-retained TKA advocate maintaining the PCL to increase stability and promote femoral rollback, thereby enhancing the patient’s ability to climb stairs,^12-14 whereas those who favor the PCL-sacrificed TKA highlight studies in which patients with a sacrificed PCL have a greater postoperative range of motion.^14-16 However, no difference in clinical or radiologic outcomes of the 2 TKAs has been shown.^17-19 Therefore, the most important factor of TKA is not whether PCL is sacrificed, but whether there is accurate alignment of the joint components and balancing of the soft tissues. Several studies have reported that sacrificing the PCL leads to larger flexion-extension gaps^1-5 because the major posterior ligamentous restraint of the knee has been removed. Mihalko and Krackow²⁰ showed that a major result of PCL sacrifice is the creation of a larger flexion gap. This result provides insight into relative joint line changes that can occur after PCL sacrifice. Matsueda et al²¹ showed in a cadaver study that the release of the PCL in conjunction with a medial or lateral release led to a mean increase of a 10.9-mm gap in 90° flexion and a 6.6-mm gap in extension.

However, all studies were performed in relatively less severely deformed knees and excluded knees with severe varus/valgus deformity or flexion contracture in which a PCL-sacrificed prosthesis might have some benefits. Few studies have evaluated the medial-lateral gap changes after PCL cutting. In our study, we used improved tensioning devices and an image-free navigation system. Through the tensioning device and navigation system, we could check the medial-lateral gaps on 90° flexion and extension before and after PCL resection.

After cutting the PCL, the flexion gap increased significantly compared with the extension gap. These results were similar to those of other studies.^1-5,20,21 In terms of medial-lateral gap change, there was only a 0.9- and 1.1-mm difference at full extension and 90° flexion, respectively. These results showed that flexion contracture and varus deformity were not corrected significantly after PCL cutting. Only 1° of correction of varus deformity and further extension is expected from PCL cutting. Change of gaps (medial, lateral and flexion, extension) led to change of femoral component size and rotation, polyethylene insert, and femoral bony resection level.

In comparing femoral component planning before and after PCL resection, the mean external rotation of the femoral component decreased by 1.6° (from 4.8° to 3.2°), polyethylene insert thickness increased by a mean of 1.2 mm, and there was a tendency for increase in the femoral component size, although the sizes varied depending on the patient’s own soft tissue status. In 83.4%, the following 3 kinds of changes occurred after PCL resection. In 9 cases (30%), the femoral component size increased by 1 size and the thickness of the polyethylene insert did not change. In 8 cases (26.7%), the size of femoral component and the thickness of polyethylene insert did not change; however, the amount of posterior femoral resection decreased. In 8 cases (26.7%), the size of the femoral component did not change, although thicker polyethylene was planned with slight increase of distal femoral cutting.

The overall amount of femoral bone resection was more suitable after PCL resection than in PCL-preserved cases. The mean posterior femoral bone cutting amount was decreased to 11.4 mm on the medial side and 8.3 mm on the lateral side, which was similar to the implant thickness, compared with 14 mm on the medial and 9.5 mm on the lateral side in cases of preserved PCL. However, the mean amount of distal femoral resection was not significantly different before and after PCL resection.

Conclusion

After PCL cutting in severely deformed knees, flexion gap increased significantly (4.5 mm on the medial, 3.4 mm on the lateral); however, extension gap and correction of varus deformity were not significant (1.2 mm on the medial, 0.3 mm on the lateral, and mean 1° of varus deformity correction). These gap changes led to alteration of planning for amount of rotation, component size, and thickness of the polyethylene insert to achieve well-balanced flexion-extension gaps. The mean external rotation of the femoral component decreased by 1.6° (from a mean of 4.8° to a mean of 3.2°). In 9 cases (30%), the femoral component size increased by 1 size without change of the thickness of polyethylene insert. In 8 cases (26.7%), the size of the femoral component and the thickness of the polyethylene insert did not change; however, the amount of posterior femoral resection decreased and was more adequate. In 8 cases, the size of the femoral component did not change; however, thicker polyethylene was planned with slightly increased distal femoral cutting.

References

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Authors

Drs Park (Sang-Jin), Seon, Park (Ju-Kwon), and Song are from the Department of Orthopedic Surgery, Chonnam National University Hwasun Hospital, Jeonnam, Korea.

This study was supported by a grant (CRI09032-1) from Chonnam National University Hospital Research Institute of Clinical Medicine.

Dr Song is a consultant for B. Braun Aesculap. Drs Park (Sang-Jin), Seon, and Park (Ju-Kwon) have no relevant financial relationships to disclose.

Correspondence should be addressed to: Ju-Kwon Park, MD, Center for Joint Disease, Chonnam National University Hwasun Hospital, 160, Ilsim-Ri, Hwasun-Eup, Hwasun-Gun, Jeonnam, 519-809, Korea.

doi: 10.3928/01477447-20090915-54