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Effect of Posterior Condylar Offset on Cruciate-retaining Mobile TKA

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Abstract

The objective of this article was to evaluate the effect of the change of posterior condylar offset to range of motion (ROM) and clinical results after computer-assisted cruciate-retaining mobile-bearing total knee arthroplasty (TKA). A total of 111 knees underwent cruciate-retaining mobile-bearing TKAs under computer-assisted navigation from January 2005 to September 2007. All cases were primary osteoarthritis and had <15° of valgus or varus deformity. We divided patients into 4 groups according to change of posterior condylar offset, which was measured by postoperative minus preoperative posterior condylar offset (group 1: <-2 mm; group 2: -2-0 mm; group 3: 0-+2 mm; group 4: >2 mm). Preoperative age, thigh girth, body mass index, flexion contracture, further flexion, Hospital for Special Surgery (HSS) score, Knee Society (KS) knee score, and KS functional score did not show significant difference between groups. The measured change of posterior condylar offset ranged from +3.70 to -3.95 mm with a mean value of -1.67 mm. Postoperatively, there were no statistical differences between each group on flexion contracture (P=.522), further flexion (P=.442), HSS score (P=.116), KS knee score (P=.479), or KS functional score (P=.578). We could find no significant difference between ROM or clinical results with computer-assisted cruciate-retaining mobile-bearing TKAs in the comparison of groups according to changes of posterior condylar offset.

Total knee arthroplasty (TKA) is the most effective method of treating end-stage degenerative osteoarthritic disease in the knee because it not only reduces pain but also allows for the recovery of functional status. Postoperative range of motion (ROM) is especially important in daily living activities and, in some cultures, for religious purposes.^1,2 Researchers and engineers have made substantial gains in improving ROM.^3-15 One of the challenges in improving ROM after TKA is minimizing the change of the posterior condylar offset, which is identified by Bellemans et al⁵ as the greatest tangential distance between the articular margin of the posterior femoral condyle and the extension of the posterior femoral cortex. Bellemans et al⁵ studied the posterior cruciate ligament (PCL)-retaining fixed-bearing TKA and concluded that the magnitude of posterior condylar offset correlated with the final ROM. Alternatively, Hanratty et al⁹ showed there was no statistical correlation between the change in knee flexion and the difference in the posterior condylar offset after TKA. However, their study used a cruciate-sacrificing mobile-bearing TKA. Interpretation of correlation of posterior condylar offset with postoperative ROM in these trials may have been affected by implant design or by surgical technique. Furthermore, neither study described preoperative factors affecting the results, such as ROM, body mass index (BMI), or diagnosis.

In this study, we controlled preoperative factors that influence the postoperative ROM and other intraoperative factors, such as the posterior tibial slope and the flexion gap balance, using a computer-assisted navigation system. We investigated whether changes of posterior condylar offset induced a different postoperative ROM and clinical results in cruciate-retaining mobile-bearing TKAs.

Materials and Methods

Patients and Materials

In this study, 109 patients (111 cases; 2 patients underwent surgery to both knees) underwent cruciate-retaining TKA using an image-free computerized navigation system (OrthoPilot version 4.0; B. Braun Aesculap, Tuttlingen, Germany) between January 2005 and September 2007. The inclusion criteria were primary degenerative arthritis, <15° of a coronal deformity (valgus or varus), BMI between 20 and 30, thigh girth between 30 and 40 cm, flexion contracture of 0° to 15°, and >100° of further flexion. We excluded patients with posttraumatic arthritis, previous osteotomy around the knee, rheumatoid arthritis, sagittal instability, and severe deformities. All surgeries were performed by 1 surgeon. Mean patient age at the time of surgery was 66.2 years (range, 52-72 years), and there were 6 men and 103 women. Of these, 2 women underwent bilateral knee surgery. There were 63 right-knee cases and 48 left-knee cases. The average follow-up period was 13 months (range, 10-43 months). All cases were performed using cruciate-retaining mobile-bearing knee prostheses (E.motion, B. Braun Aesculap).

Surgical Technique

We used a medial parapatellar approach via anterior midline incision.

We carried out mechanical registration in order of precedence with hip, knee, and ankle joint motion and surface registration of bony and articular landmarks, and evaluated the mechanical axis of the lower extremity.

We removed an 8-mm thick section of the lateral condyle, posterior tibial slope between 0° and 1°; if necessary, 2 mm more of bone was removed. In all cases, the posterior cruciate ligament was preserved.

We measured the flexion and extension gap with a distractor and processed the numerical value data of the joint space with respect to flexion and extension.

Based on the gap data and limb alignment, we performed femoral cutting design. At this step, we accepted <2 mm of gap unbalancing and assessed a difference of 1 size larger of the femoral component in those cases in which flexion gap was 3 mm wider than the extension gap.

After distal femoral cutting, we judged the degree of external rotation of the femoral prosthesis and carried out anteroposterior (AP) femoral bone removal.

We evaluated the flexion and extension gap, as well as the tension of the posterior cruciate ligament. If excessive tension was observed, we carried out a recession of the posterior cruciate ligament at the tibial attachment.

We performed intraoperative limb alignment under axial compression, as well as valgus and varus stress.

We performed patella trimming using electrocautery for synovectomy around the patella instead of resurfacing, and we evaluated patellofemoral tracking with the no-thumb technique.

We fixed the femoral and tibial components using cement.

Before closure of arthrotomy, we once again performed a limb alignment.

Methods

To begin the clinical trial, we controlled patient factors such as BMI, thigh girth, flexion contracture, and further flexion using records evaluated the day before surgery. For comparison of clinical results, we recorded the Hospital for Special Surgery (HSS) score, the Knee Society (KS) knee score, and a functional score obtained preoperatively by 1 examiner and postoperatively at final follow-up. Preoperative and postoperative lateral plain radiographs of the medial and lateral femoral condyle were also obtained to measure the posterior condylar offset. To reduce technical bias, we controlled the angle and distance between film cassette and x-ray beam tube, and made the magnification 120% or 1:1.2. The posterior condylar offset was measured relative to the maximal thickness of the posterior condyle as projected posteriorly to the posterior cortex of the femoral shaft. Using a picture archiving and communication system, 3 radiologists who were unaware of the study measured the posterior condylar offset 3 times in each case to minimize interobserver and intraobserver bias.

We divided patients into 4 groups according to the difference in posterior condylar offset, which was measured by postoperative minus preoperative posterior condylar offset (Figure):

1. Group 1: <22 mm; 31 patients.
2. Group 2: -2 to 0 mm; 54 patients.
3. Group 3: 0 to +2 mm; 20 patients.
4. Group 4: >+2 mm; 6 patients.

Figure: Patients were divided into 4 groups according to difference of posterior condylar offset, which was measured by postoperative minus preoperative posterior condylar offset (group 1, <2 mm; group 2, 2-0 mm; group 3, 0-+2 mm; group 4, >2 mm).

We analyzed preoperative and postoperative flexion contracture, further flexion, HSS score, KS knee score, and functional scores for each group. We used the Kruskal-Wallis test for statistical analysis, and the significance level was set at P<.05.

Results

The measured change of posterior condylar offset ranged from +3.7 to -3.95 mm, with a mean value of -1.67 mm. There were no statistical differences for BMI (P=.993) or thigh girth (P=.743) in any test group. Preoperative flexion contractures were 6.6° (group 1), 6.9° (group 2), 7.5° (group 3), and 7.5° (group 4), and further flexion was 118.9°, 124.0°, 124.1°, 120.8°, respectively. There were no statistical differences (P=.889, .682) for either group.

Preoperative HSS score, KS knee score, and functional scores were as follows, respectively:

1. Group 1: 59.04, 43.04, 43.39.
2. Group 2: 65.09, 42.20, 45.71.
3. Group 3: 58.63, 42.20, 41.00.
4. Group 4: 64.17, 45.17, 44.83.

There were no significant differences in clinical parameters for HSS score (P=.303), KS knee score (P=.421), or functional score (P=.562) (Table 1).

Postoperative flexion contracture and further flexion were as follows, respectively:

1. Group 1: 4.03°, 124.5°.
2. Group 2: 3.09°, 128.5°.
3. Group 3: 4.74°, 127.11°.
4. Group 4: 4.17° and 125.0°.

P value for postoperative flexion contracture was .522 and for further flexion was .422.

Postoperative HSS score, KS knee score, and function scores were as follows, respectively:

1. Group 1: 84.32, 80.55, 73.87.
2. Group 2: 87.53, 82.07, 77.45.
3. Group 3: 83.21, 81.11, 73.16.
4. Group 4: 90.33, 83.17, 75.83.

There were no significant differences in clinical parameters for HSS score (P=.116), KS knee score (P=.479), or functional score (P=.578) (Table 2).

There were no statistical differences between clinical results and ROM among groups with changes of posterior condylar offset in computer-assisted cruciate-retaining mobile-bearing TKA.

Discussion

Posterior condylar offset, as identified by Bellemans et al,⁵ describes the maximal thickness of the posterior condyle, as projected posteriorly to the posterior cortex of the femoral shaft. They demonstrated that a 2-mm decrease of posterior condylar offset in a cruciate-retaining TKA could result in a 12° limitation of flexion after surgery and required maintenance of posterior condylar offset. Also, Massin and Gournay¹⁴ reported that a 3-mm decrease of the posterior condylar offset reduced 10° of knee flexion, whereas a 5° decrease of posterior tibial slope could reduce knee flexion by 5°, and both should reduce 10 mm of femoral rollback using a bone saw. However, there were limitations in the study; for example, the researchers did not consider the amount of ligament balancing, the amount of tension of collateral ligaments that was used, or the interposition of soft tissue resulting from the use of bone saws that could influence the in vivo ROM. Thus there are definite differences between the study results and direct application in a clinical situation.

In our study, we identified no statistical differences between the change of posterior condylar offset and the clinical results, including ROM in the setting of ligament balance with the navigation system and constant posterior slope. We believe that our clinical study is more valuable in controlling intraoperative factors that affect postoperative ROM.

In clinical trials, Kim et al¹⁰ reported no statistically significant differences in postoperative knee score or ROM between standard-type cruciate-sacrificing conventional TKA and high-flexion type TKA, reflecting the concept that ROM increases with increases in posterior condylar offset. There was a 2-mm increase of posterior condylar offset in the high-flexion group, but it did not induce an increased ROM.

Hanratty et al⁹ showed there was no statistical relationship between the changes of posterior condylar offset and knee flexion after cruciate-sacrificing mobile-bearing TKA. Their results are similar to ours; however, they used cruciate-sacrificing TKA with a rotating type meniscal bearing that might produce a larger flexion gap compared to a cruciate-retaining TKA. Therefore, there was a relatively wide range of posterior condylar offset change, with a maximum of 6-mm reflected in Hanratty et al’s study.⁹ We used cruciate-retaining TKA, which produces a smaller flexion gap due to the tethering effect of the posterior cruciate ligament, and we used an AP gliding mobile-bearing insert. Our results, which showed no differences between the change of posterior condylar offset and the clinical results (including ROM), might be affected by a relatively narrow range in the change of posterior condylar offset, within ± 4 mm (range, +3.70 to -3.95 mm).

Other studies^9,14 report on the relationship between the posterior slope and ROM. Massin and Gournay¹⁴ reported that a 5° decrease of the posterior tibial slope could reduce knee flexion by 5°. Hanratty et al⁹ attempted to restore the patient’s anatomic tibial slope by 5° to 9°. However, we thought that the posterior tibial slope could significantly influence postoperative ROM and should be constantly controlled for evaluation of the exact effect of change of the posterior condylar offset. We set the posterior tibial slope at 0° to 1° using the navigation system. Although this range of posterior tibial slope could negatively affect the flexion gap, our results showed no influence on postoperative ROM. These results may be attributed to the AP gliding mobile-bearing polyethylene insert with unique implant design in which the tibial base plate has a 3° posterior slope or to the use of strict ligament balancing using the navigation system.

When the AP distal femoral bony resection was performed, we expected that excessive posterior femoral bone cutting would result because of the anterior referenced distal femur AP cutting used in the TKA instrumentation and because of the gap technique principle applied under computer-assisted navigation guidance. This would have significantly influenced any change of posterior condylar offset and postoperative ROM. However, a relatively narrow range of changes of the posterior condylar offset was obtained in our results. In part, this result may be because we tried to restore preoperative femoral AP size by cutting more of the distal femur and because we used a 2-mm thicker tibial insert instead of using a one-size-larger femoral component when flexion gap was larger than extension gap by 3 mm.

Overall, we believe the navigation system was beneficial for constantly maintaining the tibial posterior slope, maintaining the gap balance at 2 mm through intraoperative real-time correction, reducing the effect of change of posterior condylar offset, and improving the postoperative ROM.

Our study has some limitations. We were not able to control some factors that were affecting postoperative ROM (eg, length or strengthening of the quadriceps muscle, patient’s rehabilitation efforts), although we did control other patient factors such as BMI, preoperative ROM, and thigh girth. Furthermore, this study included a relatively small number of patients, especially in group 4. This resulted in low statistical power and limited statistical effectiveness. Nevertheless, we think that our study is meaningful in terms of evaluating the effect of a ligament-balancing technique on the change of posterior condylar offset using a navigation system and the relationship between clinical results including ROM and change of posterior condylar offset in computer-assisted cruciate-retaining mobile-bearing TKAs.

Conclusion

We investigated the role of changes in posterior condylar offset to postoperative ROM and clinical results. To reduce study bias, we tried to control preoperative factors and intraoperative factors affecting postoperative ROM by using a computer-assisted navigation system and by filtering demographic data.

In our study, there were no clinical differences in postoperative ROM or clinical results among groups with changes in posterior condylar offset. We conclude that change of posterior condylar offset alone by as much as 4 mm does not affect clinical results or postoperative ROM when there is an acceptable balance of gap in computer-assisted cruciate-retaining mobile-bearing TKAs.

References

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Authors

Drs Seo, Ha, Kim, and Choi are from Pusan Paik Hospital, College of Medicine, Inje University, Busan, Korea.

This study was supported by a grant of the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea (A084120).

Dr Seo received a grant from Inje University in 2008. Drs Ha, Kim, and Choi have no relevant financial relationships to disclose.

Correspondence should be addressed to: Seung-Suk Seo, MD, PhD, Department of Orthopedic Surgery, Pusan Paik Hospital, 633-165 Gaegeum-Dong, Jin-Gu, Busan, Korea.

doi: 10.3928/01477447-20090915-59