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Unicondylar Knee Arthroplasty in Middle-aged Patients: A Minimum 5-Year Follow-up

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Abstract

The purpose of this study was to report the results of unicompartmental knee arthroplasty in patients 60 years or younger for a period long enough to be statistically significant. In a cohort of 1173 unicompartmental knee prostheses implanted between 1991 and 2006, 161 knees from 158 patients 60 years or younger at the time of surgery who had 5 to 14 years of follow-up were reviewed. The Genesis unicompartmental knee replacement (Smith & Nephew, Memphis, Tenn) with the metal-back or full-poly design was implanted in these patients. The Knee Society score at most recent follow-up was 94.02, and average function score was 93.76. Survivorship for the entire cohort was 94.50% at 10 years and 88.48% at 12 years.

During the past 5 years, the controversial unicompartmental knee prosthesis regained a choice position in the armamentarium of surgeons treating unicompartmental knee osteoarthritis. Although unicompartmental arthroplasty is now accepted by most of the orthopedic community for treating elderly patients, the procedure is not thought to be necessarily suitable for highly active middle-aged patients¹ because of the higher revision rate² in this younger population. However, for active middle-aged patients not considered appropriate candidates for either total knee replacement or tibial osteotomy, some surgeons prefer unicompartmental replacement because of the quality of the long-term results.^3-5 Furthermore, it appears that the outcome of unicompartmental knee arthroplasty is better than that of tibial osteotomy at long-term follow-up,⁶ and that revision of unicompartmental prostheses allows one to achieve results identical to primary total knee replacement (TKR).⁷ Outcomes do not appear to be as good for TKR performed after a tibial osteotomy.⁸

Materials and Methods

The procedure was used to treat unicondylar arthritis in contraindications to osteotomy in the following cases: women older than 50 years; absence of tibial varus curvature in the varus deviations of purely intraarticular origin; and reduction of the joint space by >50% as noted by Lootvoet et al⁹ on the full-limb radiography without any magnification (gonometry). A flexion deformity of up to 15° did not necessarily contraindicate unicompartmental arthroplasty, provided the deformity could be corrected through intra-articular notchplasty and removal of the anterior tibial bony block. Exclusion criteria were inflammatory disease, overweight by more than 100 kg, and deformities that could not be corrected passively at stress radiography. Lateral ligament insufficiencies also were excluded. Absence of the anterior cruciate ligament (ACL) had been considered a classic contraindication, in accordance with Deschamps and Cartier¹⁰; however, such patients may again become candidates for unicompartmental replacement after surgical correction of the instability through intra-articular reconstruction or extra-articular procedures. The examination preoperatively or intraoperatively may persuade the surgeon to change course and proceed with a TKR rather than a unicompartmental replacement in certain situations. One hundred sixty-nine unicompartmental knee arthroplasties were performed in patients aged 60 years or younger. Eight patients were lost to follow-up. One hundred fifty eight were re-examined (3 bilateral).

The study included 161 knees (158 patients; 81 men). The mean age was 53 years (range: 30-60 years). The mean follow-up period was 9 years (range: 5-14 years). The sample included 130 varus knees and 31 valgus deviations. The primary etiology was primary osteoarthritis in 129 knees; post-traumatic in 22 knees (including 13 medial meniscectomies, 7 lateral meniscectomies, and 2 intra-articular fracture consequences); 9 medial condyle avascular necrosis in 9 knees; and septic arthritis sequelae in 1 case. The tibial implants used in the study included 92 2-mm-thick titanium metal-backed tibial plates with removable poly inserts (32 cemented and 60 cementless with hydroxyapatite coating and screw fixation), and 69 all poly-cemented tibial implants. The ACL was present in 142 knees and was absent or insufficient in 19 knees. Preoperatively, all patients underwent radiographic evaluations, including frontal and lateral weight-bearing radiographs in maximal extension, full long leg limb radiograph without magnification (gonometry), frontal view of the patella (Merchant type) at 30° and 60° of flexion, Schuss radiographs, and stress radiographs at 15° flexion in a prone position. Postoperatively, only frontal and lateral weight-bearing radiography in maximal extension and frontal view of the patella are needed.

Surgical Technique

Both patellar dislocation and minimally invasive surgical techniques were used. Lateral patellar release was systematically performed for the lateral unicompartmental knee replacements and in the presence of localized pain in the lateral patellar facet in medial unicompartmental knee replacement (86 knees).

For the tibial cut in genu varum, at the tibial level, the given cutting angle in the coronal plane was precalculated and corresponded to the varus tibial bow. It is thus perpendicular to the tibial epiphysial axis. When the tibial varus bow exceeds 6°, use of the unicompartmental knee prosthesis alone is contraindicated because of potential future tibiofemoral subluxation. A posterior tibial slope of 3° without exceeding this value was used in this series. For the tibial cut in genu valgum for a lateral compartment unicompartmental knee arthroplasty, the tibial cut in the coronal plane and the posterior slope must have a value of 0° because of the particular convex anatomy of the lateral tibial plateau.

At the condylar level, the only resurfacing technique used strictly stops at the subchondral bone, as recommended by Marmor.¹¹

In accordance with Tabor et al,³ no ancillary system, however sophisticated, exempts a surgeon from performing a unicompartmental arthroplasty after rigorous intraoperative inspection and evaluation after the trial implants have been placed.

Results

The Knee Society rating scale was used to evaluate the results, separating knee score from patient function. The total overall score at the most recent follow-up was 187.78, compared with 93.28 preoperatively. The postoperative knee score averaged 94.02 and was better for valgus than for varus knee (95.42 vs 92.62) according to Pennington et al.¹² and was also better for active men (95.41) than for active women (92.63). The postoperative function score was 93.76 and is again higher in men (96.20) than in women (91.32). For lateral unicompartmental knee arthroplasty, the average scores were 95.28 for men and 92.23 for women. The relatively small difference between knee and function score compared with unicompartmental knee arthroplasty statistics in older patients¹³ is easily explained by the lower frequency of other arthritic abnormalities at the joint or spinal level in that age group. The analysis of functional score data enables objective assessment of significant differences according to the type of implant used. There is indeed a significant mean positive difference of 5 points in favor of full-poly implants vs metal back in female patients compared with male patients. This pattern was particularly marked for axial varus deformities where the negative difference was 7 points for women with metal-back prosthesis compared with 2 for men, with minor differences in the case of lateral unicompartmental knee arthroplasties, regardless of gender. On the other hand, the comparison between cementless hydroxyapatite-coated and cemented metal back shows a score 3 points higher for cementless prostheses compared with cemented implants.

Twenty-three patients practiced sports at a fairly high level (including jogging, gymnastics, tennis, boxing training, and soccer refereeing) or at a more moderate level (hiking, walking, golfing, and cycling) prior to joint impairment. Nineteen resumed sports at the same level (Figure 1). Four patients did not return to a high level of activity, 1 because of a heart condition, and 3 because of distant joint deterioration. Nineteen patients who had strenuous jobs (farming, construction work, etc) prior to joint impairment returned to work in their previous capacity and were able to maintain that activity level.

Complications

No sepsis or thromboembolic phenomenon occurred in this series. Eight complications required implant revision with a mean postoperative follow-up of 6 years. Five of these knees had undergone prior surgery. Complications consisted of 5 cases of tibial loosening, 3 in multioperated women with osteoporotic bone and metal backed implant, and 2 in men. One loosening was attributable to inappropriate indication, and the other occurred in an active farmer with a metal-back cemented implant at 11 years’ follow-up without any special cause, maybe a very high level of physical activity for 10 years. One case of secondary instability in varus occurred, also attributable to inappropriate indication for unicompartmental knee arthroplasty, considering the too severe initial varus bow. One case of deterioration of the opposite tibiofemoral compartment occurred in a young woman 5 years after undergoing unicompartmental knee arthroplasty following a failed undercorrected tibial osteotomy. One case of unexplained painful joint occurred on a patient with a unicompartmental cementless implant.

Of 8 patients who required revision surgery, 5 underwent major revisions with TKR (4 cemented) (Figure 2), and 3 underwent minor revisions (two standard exchanges of the tibial plateau and one complementary unicompartmental arthroplasty on the opposite tibiofemoral compartment). All revisions were as uncomplicated as a primary TKR. No allograft was required, and all defects were filled with the patient’s bone.

Radiographic Results

The average postoperative alignment for medial arthroplasty was 3° of mechanical varus. The alignment for the lateral arthroplasty averaged 4° of mechanical valgus. Evaluation of the opposite healthy compartment showed a normal radiographic appearance except in 2 cases with 1 complete joint space narrowing (previous high tibial osteotomy case and previous septic arthritis) and 2 knees with limited degenerative changes. No lucent lines were present at the femoral level. One stable 2-mm lucent line was identified under a metal-back implant 13 years after implantation in a multioperated knee. Patellofemoral skyline view showed no significant abnormalities.

Polyethylene wear was measured by comparative frontal and sagittal radiography performed immediately postoperatively and at the most recent follow-up. Average wear was <1 mm including that for 2 obese patients.

Five active men with metal back unicompartmental knee arthroplasty have shown above-average wear of 3 mm in 2 patients and 2 mm in the 3 other patients.

Survivorship Analysis

A survival table was created in a manner described previously.^14,15 The patients were grouped in two intervals of 1 year, with failure defined as revision or need for revision. The table was carried out until 12 years, after which the confidence limits became too low. As with all such survivorship analyses, the patients lost to follow-up evaluation were assumed at each interval to be doing no better or worse than the cohort still available for study. As such, the survivorship was calculated to be 94% at 10 years, 92% at 11 years, and 88% at 12 years (Figure 3).

Discussion

The review of this series of patients 60 years or younger leads to several observations.

Although lower than the curve of patients operated at an older mean age,^1,11,16 survival curve in the present series is promising given their activity level. It shows a survival rate of 94% at 10 years, which drops to 88% at 12 years. The results for lateral unicompartmental knee arthroplasty proved superior to medial unicompartmental knee arthroplasty, which concurs with Pennington et al.¹² Five of the necessary revisions occurred in multioperated patients for whom unicompartmental knee arthroplasty was not the ideal choice because of inappropriate indications, but was preferable to total arthroplasty given their young age.

Despite activity level, polyethylene wear did not exceed 1 mm, except for wear in 5 knees in men with a mean follow-up of 10 years. No lucent lines exceeding 1 mm were observed in women, two-thirds of whom had received an all-poly tibial component. Even in view of their relatively lower activity level, this observation, combined with that of three surgical revisions because of loosening in women who had metal-back tibial implants with less resistant tibial bone matrix, led to the decision to use all-poly tibial components for women in the future. The quality of results for cementless and osseo-integration tibial implants will lead us to increase future indications for this type of implantation in men with potential high levels of activity to minimize the risk of tibial loosening in accordance with the conclusions of Epinette and Edudin.¹⁷

The absence of complications during revision surgery confirms recent opinions^1,5,7 and invalidates older ones.¹⁸ This finding is linked to 4 main parameters: 1) using a minimal amount of polymer for cemented implant; 2) using a resurfacing condylar implant supported by the subchondral resistant bone as opposed to condylar resection designs (this probably explains why only one condylar loosening occurred in the 1173 unicompartmental arthroplasties performed between 1993 and 2006, regardless of age. In collaboration with Grelsamer and Cartier,¹⁹ we had already reaffirmed that a unicompartmental knee arthroplasty should not be considered as “half a total knee”); 3) conserving the condylar anatomy except for the posterior cut making revision total knee arthroplasty easier; and 4) obtaining yearly follow-up radiography for patients having undergone unicompartmental knee arthroplasty to be able to act before implant sinkage or joint metallosis.

The absence of the ACL does not appear to be a formal contraindication to unicompartmental knee arthroplasty, provided it is possible to differentiate known previous post-traumatic instabilities, which may have been concealed and stabilized by the wear defect of the tibial plateau and the ACL progressive deterioration inside the intercondylar notch caused by osteophytes. With regard to the 19 cases in which the ACL was absent, complementary ligamentoplasties were performed in 14 patients, consisting of 7 intra-articular, and 7 extra-articular. Results were similar to those of primary arthroplasties, in accordance with Pandit et al.²⁰ The absence of the ACL in the 5 remaining patients with low activity level did not alter the quality of the functional result at 9-year follow-up for that particular group with a tibial sagittal cut at 0° slope.

Conclusion

Under no circumstances should the use of a unicompartmental knee arthroplasty in middle-aged patients replace tibial osteotomy in its elective indications. When a tibial osteotomy is contraindicated, it seems illogical to use it excessively based solely on the contention that the patient is too young for a unicompartmental knee arthroplasty. A tibial osteotomy will statistically lead to a less sustainable functional outcome and to revision TKR, which tends to be more complex in patients of relatively young age. Furthermore, even when the tibial osteotomy may be a solution, a unicondylar arthroplasty can be preferred to shorten the immobilization period that can impair patients’ professional activity.

References

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Authors

Drs Cartier, Khefacha, and Sanouiller are from the Hartmann Knee Institute, Clinique Hartmann, Neuilly-sur-Seine, France, and Dr Frederick is from the Knee Center in Rolla, Mo.

Drs Cartier and Frederick are consultants for Smith & Nephew and have leadership roles to disclose. Drs Khefacha and Sanouiller have no affiliation with materials mentioned herein to disclose.