Precise outcome evaluation is mandatory to improve analysis of the results of knee replacement procedures. Patients’ expectations toward surgery and activity levels have increased with changes in patient populations and improvement of surgical results. It is difficult, however, to accurately assess outcomes because objective evaluation of patient function performed only by a surgeon remains highly inaccurate. New methods of objective evaluation after unicompartmental knee arthroplasty have been developed. These devices provide information about range of motion and patient function during daily activities. This article provides up-to-date information concerning the different tools of function evaluation after unicompartmental knee arthroplasty.
Key points
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Over the past decade, progress in the UKA ancillary and technique had increased the results of the procedure in terms of survivorship and satisfaction even for young patients.
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To access functional outcomes of UKA, different dimensions must be taken into consideration. Some refer to objective elements, such radiographic outcomes, kinematics or survivorship.
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Some are more subjective and individual parameters of our patients should be evaluated by specific scoring systems, including both patient’s and surgeon’s experience of the UKA.
Introduction
Management of osteoarthritis of the femorotibial compartments in young subjects is controversial. In cases of medical treatment failure, surgery can be required to relieve pain and restore function. Available surgical possibilities include conservative surgical treatments with limited arthroscopic débridement or valgus high tibial osteotomy, which is effective in certain precise indications. When conservative treatments are no longer efficient, prosthetic replacement must be considered: unicompartmental knee arthroplasty (UKA) or total knee arthroplasty (TKA). The population of subjects ages less than 60 years includes active patients whose management must take particular needs into consideration: functional recuperation, resumption of sports activities, and the lifespan of the implants are 3 of the specific problems added to limiting extension of osteoarthritis to the neighboring compartments. Over the past decade, progress in the ancillary instrumentation used to implant these unicompartment prostheses associated with better patient selection has accelerated functional recuperation and increased the satisfactory clinical results based on the classical scores and satisfactory implant survival, even if the wear rates seem higher in this age group. The study of implant survival has, however, thus far been insufficient to demonstrate the value of an intervention, particularly in a population that is young and active, whose quality of life can be strongly related to the condition of the knee. To access the functional outcomes of UKA, different dimension must be taken into consideration. The aim of this article is to provide up-to-date information concerning the different tools of function evaluation after UKA, including (1) clinical and radiographic analysis, (2) knee kinematics evaluation, (3) survivorship of implant (medial or lateral after primary arthritis, osteonecrosis, or trauma), and (4) subjective results based on patient-rated outcomes questionnaires, including knee-related quality-of-life or general knee-related quality-of-life questionnaires.
Introduction
Management of osteoarthritis of the femorotibial compartments in young subjects is controversial. In cases of medical treatment failure, surgery can be required to relieve pain and restore function. Available surgical possibilities include conservative surgical treatments with limited arthroscopic débridement or valgus high tibial osteotomy, which is effective in certain precise indications. When conservative treatments are no longer efficient, prosthetic replacement must be considered: unicompartmental knee arthroplasty (UKA) or total knee arthroplasty (TKA). The population of subjects ages less than 60 years includes active patients whose management must take particular needs into consideration: functional recuperation, resumption of sports activities, and the lifespan of the implants are 3 of the specific problems added to limiting extension of osteoarthritis to the neighboring compartments. Over the past decade, progress in the ancillary instrumentation used to implant these unicompartment prostheses associated with better patient selection has accelerated functional recuperation and increased the satisfactory clinical results based on the classical scores and satisfactory implant survival, even if the wear rates seem higher in this age group. The study of implant survival has, however, thus far been insufficient to demonstrate the value of an intervention, particularly in a population that is young and active, whose quality of life can be strongly related to the condition of the knee. To access the functional outcomes of UKA, different dimension must be taken into consideration. The aim of this article is to provide up-to-date information concerning the different tools of function evaluation after UKA, including (1) clinical and radiographic analysis, (2) knee kinematics evaluation, (3) survivorship of implant (medial or lateral after primary arthritis, osteonecrosis, or trauma), and (4) subjective results based on patient-rated outcomes questionnaires, including knee-related quality-of-life or general knee-related quality-of-life questionnaires.
Objective evaluation of unicompartmental knee arthroplasty
Clinical Evaluation
The clinical evaluation is classical and based on the evaluation of the pain, range of motion, the stability of the knee, and the function of the patient. The Knee Society Score was classically used to compile these results. This evaluation most of the time was not sufficient for patients with a UKA due to the high demand and the lack of subjective evaluation. Furthermore, younger patients operated on with a UKA place a high demand on their knee when performing sport activities. The new Knee Society Score includes items concerning the subjective feeling of patients not only during daily living and standard activities but also during advanced and recreational activities. The validation of this new Knee Society Score for UKA is currently under process and will be an effective tool to better address patient expectations.
Radiologic Evaluation
Classically, radiographic evaluation ( Figs. 1–3 ) has been based on long-leg radiographs and on anteroposterior (AP), lateral, and skyline radiographs of the knee to obtain measurement of preoperative and postoperative hip-knee-ankle angles and to classify lower limb alignment (described by Kennedy and White ). The lower limb alignment is considered correct when the mechanical axis passes through either the medial part of the medial tibial plateau (zone 2) or the central part of the tibia (zone C) ( Fig. 4 ). During this evaluation, alignment and position of the components must be assessed on screened AP and lateral radiographs and the presence, extent, or progression of femoral or tibial radiolucencies ( Figs. 5 and 6 ) according to the Knee Society roentgenographic score is recorded. Progression of osteoarthritis has to be evaluated in the uninvolved compartment on the AP radiographs and in the patellofemoral joint on skyline views according to Berger 4-point scale. According to this scale, grade 1 radiologic change is defined as no measurable loss of joint space but with changes, such as osteophyte formation. Grade 2 changes are defined as up to 25% loss of joint space, grade 3 up to 50%, and grade 4 as more than 50%.
Knee Kinematics After Unicompartmental Knee Arthroplasty
Fluoroscopy has been described as an effective tool to evaluate in vivo motions of knee replacement. This peculiar device may help surgeons understand range of motion after UKA, by giving dynamic information during flexion-extension movement or during gait ( Figs. 7–10 ).
Some fluoroscopic analyses, focused on the kinematics of posterior cruciate–retaining and posterior stabilized total knee replacements, determined that a paradoxic anterior slide occurs during gait and deep flexion in subjects with total knee replacement. From these studies, it has been hypothesized that the anterior cruciate ligament plays an important role in knee kinematics. Argenson and colleagues reported, in 2002, the results of their series of 70 medial UKA and 3 lateral UKA kinematics in knees in which the anterior cruciate ligament was intact at the time of the surgery. All the knee arthroplasties were judged clinically successful with a Hospital for Special Surgery score of greater than 90 points, with no ligamentous laxity or pain. Under fluoroscopic surveillance, each subject was asked to perform successive weight-bearing deep knee bends to maximum flexion. Subjects with a medial unicompartmental knee replacement had only minimal motion during flexion, and kinematic patterns varied among the subjects. The average AP contact position was 0.0 mm at full extension, −2.1 mm at 30° of knee flexion, −2.1 mm at 60° of knee flexion, and −0.8 mm at 90° of knee flexion. Seven of the 17 subjects had normal posterior femoral rollback of the medial condyle from full extension to 90° of knee flexion. At full extension, 8 of the 17 subjects had a too anterior contact of the medial condyle. In other hand, patients with a lateral unicompartmental knee replacement had posterior femoral rollback. The average AP contact position between implants for these subjects was −1.95 mm at full extension, −5.1 mm at 30° of knee flexion, −6.7 mm at 60°of knee flexion, −6.0 mm at 75° of knee flexion, and −4.5 mm at 90° of knee flexion. There was increased rollback of the lateral condyle in the posterior direction from full extension to 60° of knee flexion. From 60° to 75° of knee flexion, minimal motion of the lateral condyle was detected on the average, and from 75° to 90° of knee flexion, an average of 1.5 mm of anterior motion was detected. With both medial and lateral UKA, there were variable kinematic patterns, with an anterior slide with increasing knee flexion (especially with medial UKA) occurring at either 30°or 60°of knee flexion. The results of this study suggested that progressive laxity of the anterior cruciate ligament may occur over time. In this study, it seemed that 8 subjects with medial UKA had an anterior contact position at full extension, which may lead to the hypothesis that the anterior cruciate ligament was not functioning properly, because the anterior cruciate ligament was unable to provide an anterior constraint force with the necessary magnitude to thrust the femur in the anterior direction at full extension. This suggests inconsistent function of the anterior cruciate ligament after UKA and may account, at least in part, for the premature polyethylene wear occasionally seen after UKA.
Implant Survival
Survivorship of UKA is classically based on the evaluation of the number of complete or partial revision. Because the indications are different with high tibial osteotomy or TKA, the survivorship of these 3 procedures cannot be directly compared. Reliable function and good survival have been reported for TKA in younger patients, and this type of treatment has also been advocated for unicompartmental osteoarthritis. Recently, Morgan and colleagues reported a 96% 12-year survival in a series of 63 young patients (mean age 50.7 years) treated with TKA for osteoarthritis. Mont and colleagues also reported only 1 failure because of polyethylene wear in a series of 31 knees in patients under 50 treated with a TKA for osteoarthritis, with a mean follow-up of 86 months. According to Hanssen and colleagues, despite the good clinical results, TKA should continue to be considered with caution for young patients because of the issues related to the eventual need for a revision. In a study by Pennington and colleagues, 2 of 45 UKAs (4.5%) were revised for polyethylene wear in patients under 60. Price and colleagues, in a multicenter study of the Oxford UKA, compared 512 patients older or equal to 60 and 53 patients younger than 60 and showed that this implant functions well and is durable in patients younger than 60, although the survival was lower for this group (91% at 10 years in the <60 group vs 96% in the >60 group).
In another recent report, 93% of patients successfully returned to regular sporting and physical activities after a UKA, but the patients were older, with a mean age of 64.
Medial UKA
Fixed-bearing UKA
Several studies compared the results between fixed-bearing and mobile-bearing UKA. Although mobile-bearing congruent polyethylene may be an alternative to fixed bearing, the risk of dislocation remains higher than in fixed-bearing UKAs in the young patient group. It has been demonstrated in a biomechanical study that running and jumping produce surface loads that exceed the limits of polyethylene resistance. To recommend specific activities after a UKA, factors, such as wear, joint load, and the type of prosthesis, must be taken into account for each patient. Based on the authors’ results and previous reports, decreasing wear seems the main factor in improving long-term results of UKA. The diagnosis of polyethylene wear was made clinically and radiologically. Two major symptoms were observed with polyethylene wear ( Fig. 11 ), medial knee pain without signs of inflammation and progressive nontraumatic frontal instability. There was no clinical or biologic sign of infection. Physical examination is important to confirm the development of frontal instability in these patients. When these symptoms occurred, the authors’ performed weight-bearing and varus and valgus stress radiographs to confirm wear and to look for osteolysis. When there was no evidence of infection, loosening or osteolysis exchange of the polyethylene insert was undertaken and the wear was mainly found on the posterior aspect (see Fig. 3 ; Figs. 12 and 13 ).
In 2011, the authors and colleagues presented the results in term of survivorship of fixed-bearing UKA versus mobile-bearing UKA in young patients. The authors retrospectively reviewed 75 patients (79 knees) with a fixed-bearing UKA and 72 patients (77 knees) with a mobile-bearing UKA operated on between 1989 and 1992. The mean age of patients was 63 years; gender and body mass index (26 kg/m 2 ) were comparable in the 2 groups. Knee Society function and radiographic scores were determined and survival determined. The minimum follow-up was 15 years (mean 17.2 ± 4.8 years; range, 15–21.2 years). Radiographically, the number of overcorrections and the number of radiolucencies were statistically higher in the mobile-bearing group (69% vs 24%) but at final follow-up, considering revision for any reason, 12 of 77 (15%) UKAs were revised (for aseptic loosening, dislocation, and arthritis progression) in the mobile-bearing group and 10 of 79 (12%) in the fixed-bearing group (for wear and arthritis progression). This difference did not reach the significance level.
Mobile-bearing UKA
Mobile-bearing UKA, using a specific design, Oxford Partial Knee System (Biomet, Warsaw, Indiana), has shown a recent increase in use. The challenges with mobile-bearing design include technical issues of ligament balancing with the potential risk of bearing dislocation. Several studies have suggested excellent long-term survivorship. Murray and colleagues reported 98% cumulative prosthetic survivorship at 10-year follow-up. Svärd and coworkers retrospectively evaluated 124 mobile-bearing UKA, with a cumulative survivorship of 95%. They noted that balancing the flexion and extension gaps was challenging. Price and colleagues, using the same implant, found a 15-year survivorship of up to 92%. They noted, however, a high frequency of complete radiolucent lines around the tibial component in half of the tibial components. Other studies, however, suggest contradictory results with similar mobile-bearing implants. Lewold and colleagues, in evaluating the Swedish Knee Arthroplasty register, found 90% survivorship at 5-year follow-up, using the same Oxford implant. Vorlat and colleagues reported an 84% survival of 149 consecutive Oxford partial knees at 5.5 years. In a US Investigative Device Exemption study, 125 Oxford UKAs were followed at 8 sites. This was a prospective multicenter study and found at 7 years that there was only an 80.6% survivorship, and the clinical success rate was only 74.2%. There are 3 comparison studies evaluating fixed-bearing and mobile-bearing designs. Confalonieri and colleagues found no statistical difference in outcome. Gleeson and colleagues found 3 bearing dislocations in the mobile-bearing group and 4 additional revisions. In the fixed-bearing group, there were only 3 revisions. They found that the Oxfords had a high reoperation rate and the St. Georg Sled (Waldemar Link, Hamburg, Germany) device achieved better pain relief. The functional results of both implants were similar. These comparative studies show no consensus between mobile bearing and fixed bearing in regards to function, success, or recovery, and a mobile-bearing design has the potential complication of bearing dislocation.
Lateral UKA
Midterm and long-term studies suggest reasonable outcome at 10 years, with survivorship greater than 95% of UKA performed for medial osteoarthritis or osteonecrosis, but few long-term follow-up data are available for lateral UKA. The investigators of 1 small series of UKA in the lateral compartment reported only 1 failure out of 19 patients at 89 months of follow-up. Recently, 2 other series reported high functional scores without revision at 5.2 years for 1 and at 12.4 years for another. Both the anatomic and biomechanical characteristics are different in each of the knee femorotibial compartments, and similar surgical treatment may not provide reproducible results when applied to a different compartment. Furthermore, UKA in the lateral compartment has been described as technically more challenging and 10 times less performed than medial UKA, thus representing fewer than 1% of all knee arthroplasty procedures. These facts may explain the few data available concerning outcomes of lateral UKA. In 2008, the authors presented results of a consecutive series of 39 lateral UKA. The data demonstrate lateral UKA can provide reasonable clinical and radiographic results, with survivorship at 10 and 16 years comparable to the survivorship obtained for medial UKA ( Figs. 14–18 ). Recent studies reported a low failure rate, whereas the results of older series were more controversial. Gunther and colleagues reported a 21% failure rate using the mobile-bearing Oxford unicompartmental prosthesis in the lateral compartment, with a 10% rate of bearing dislocation. This difference with the commonly reported high-functioning long-term outcomes using the same implant for the medial compartment may be explained by the amount of femoral translation of the lateral condyle whereas the medial one remains fairly stationary. When studying the in vivo kinematics of patients implanted with either a medial or lateral UKA, the authors showed an important posterior femoral translation of the lateral condyle during flexion compared with the medial one. According to these results and as a result of the biomechanical properties of the lateral compartment, fixed-bearing implants seem more appropriate. Sah and Scott reported no revision at 5 years in a group of 49 knees implanted with lateral UKA. The rate of radiolucencies observed in the authors’ series (10% of nonprogressive tibial radiolucencies) was comparable with those observed in previous series of lateral UKA at the same follow-up. Because both the anatomic and the biomechanical characteristics are different in each of the knee’s femorotibial compartments, some surgical considerations may be outlined for the lateral compartment. The rule of undercorrection of the deformity should be strictly applied on lateral UKA to avoid medial OA progression. Furthermore, the positioning of the femoral component should accommodate the femoral divergence of the lateral condyle when the knee is flexed to avoid impingement with the tibial spines when brought into extension. The mediolateral positioning of the femoral component should also avoid the excessive lateral placement in extension, which may lead to an overload of the lateral part of the tibial plateau when the knee is flexed to 30°. Additionally, internal rotation of the tibial component when performing lateral UKA accommodates the typical screw-home mechanism occurring during knee flexion, and this should be included when performing the sagittal tibial cut.
