Risk Factors for Postoperative Knee Stiffness in Patients with Anteromedial Knee Osteoarthritis Undergoing Unicompartmental Knee Arthroplasty with Cemented Prostheses: A Short-Term, Retrospective, Case-Control Study

Background

Unicompartmental knee arthroplasty (UKA) has a limited but proven indication for this procedure for the treatment of osteoarthritis of the knee involving isolated unicompartmental knee disease with normal anterior cruciate ligament (ACL) function [1]. For patients with end-stage degenerative and inflammatory arthritis who have failed conservative and nonoperative treatment, total knee arthroplasty is recommended [2]. According to Riddle et al [3], the proportion of unicompartmental knee arthroplasty was 6570 implants in 1998 and 44 990 implants in 2005. UKA grew at an average rate of 32.5% over the study period, with unicompartmental implants currently accounting for less than 8% of all knee replacement procedures. Mikkelsen et al reported that compared to total knee arthroplasty (TKA), medial UKAs has lower mortality rates, shorter hospital stays, is less invasive, has shorter operative times, higher satisfaction rates, lower infection rates, and fewer complications, and are more cost-effective for the treatment of medial anterior osteoarthritis [4]. Consequently, UKA is a surgical alternative that offers distinct advantages over other procedures [5].

UKA may not always provide satisfactory prognosis for patients who do not have a total knee arthroplasty with artificial joint implant [6]. Different cohorts have different incidences of stiffness in these types of knee joints. According to Fournier et al, the prevalence of stiffness treated with UKA was 9% at the 1-year follow-up [7]. Hurst et al revealed that 12% of patients experienced stiffness in their knee joints, which subsequently led to treatment failures [8]. Stiff knees can cause pain and limited function during daily activities, particularly when ascending or descending stairs, sitting or standing up, and tying shoelaces [9]. Meanwhile, more complex surgical procedures, such as surgical dissolution of scar tissue (arthroscopic or open surgery) or revision arthroplasty, are required in some recalcitrant cases to relieve pain and restore knee function [7]. We reviewed many studies. Knee stiffness can be disabling and is among the most common causes of patient dissatisfaction following UKA. The treatment of stiffness can range from aggressive physical therapy to revision arthroplasty [7,10–14]. Therefore, knowing how to reduce the incidence of knee stiffness after UKA has become crucial.

Researchers in the field of knee surgery are continuously investigating various factors that may contribute to postoperative knee stiffness following UKA [15,16]. Fournier et al retrospectively analyzed data from 240 patients undergoing UKA surgery. Robotic-assisted surgery was performed in 164 patients and mechanical instrumentation in 76 patients. They demonstrated that stiffness after UKA is correlated with mechanical instrumentation as a risk factor [7]. Researchers such as Saito et al have argued that inserting a component that is too thick into a narrow space can result in joint stiffness. Therefore, it is important to determine the different levels of tightness in the surrounding soft tissues after implantation [6]. The results above show that the knee stiffness after UKA is influenced by multiple factors, not just one. In this regard, it is worth noting that UKA typically exhibits a low incidence of stiffness [17,18]. Although the knee stiffness after UKA is a known complication [19], there is a lack of comprehensive literature on the potential multifactorial risks associated with this condition. This study aimed to fill this research gap by analyzing the potential factors that contribute to knee stiffness after UKA. Identifying independent risk factors for postoperative knee stiffness following UKA is crucial for developing effective treatment strategies that can improve patient outcomes. This will significantly improve patients’ quality of life after UKA and reduce healthcare costs due to postoperative stiffness, and will elucidate the mechanisms of the onset and development of postoperative stiffness in UKA.

Material and Methods

STUDY DESIGN:

This retrospective case-control study evaluated patients with anteromedial knee osteoarthritis who underwent medial UKA at our hospital between May 2017 and May 2020. Our evaluation of patients lasted 1 year. The patients were divided into 2 groups according to their prognosis: those who experienced knee stiffness after undergoing UKA and those who did not. Independent risk factors for knee stiffness after receiving UKA therapy were identified. These factors can help healthcare professionals make better-informed clinical decisions.

PARTICIPANTS:

The inclusion criteria were as follows: (1) patients who were diagnosed with anteromedial osteoarthritis and had radiological findings consistent with anteromedial osteoarthritis; (2) patients who underwent 1 UKA operations on a unilateral knee; (3) medical records and radiological examinations were available for identifying the demographic information, anatomical characteristics, and surgical procedure characteristics.

To calculate the sample size required in our study, we used the N ≥A+Bm rule. Green et al [20,21] reported that slightly more complex rules-of-thumb of the form that N ≥A+Bm show slightly better agreement with power analysis results.

The exclusion criteria were as follows: (1) patients who had been followed up for less than 1 year; (2) periprosthetic fractures occurring during patient follow-up; (3) patients who had previous knee surgery before and after UKA surgery. Based on the inclusion and exclusion criteria, 590 eligible patients were retrospectively screened and included in this study.

The study was approved by the Institutional Review Board of the No. 215 Hospital of Shaanxi Nuclear Industry and was conducted in accordance with the Declaration of Helsinki and the regulations of the Health Insurance Portability and Accountability Act (HIPAA).

The need for informed consent was waived because this was a retrospective study and all patient information was deidentified before the analysis.

It is important to note that the surgical indications for UKA include anteromedial osteoarthritis and osteonecrosis, which are often characterized by severe pain in the inner side of the knee [22].

DEMOGRAPHIC INFORMATION:

The medical records were used to extract demographic data about the patients, including age, sex, hypertension, BMI, smoking, osteoporosis, and diabetes.

CLINICAL EVALUATIONS: The preoperative Knee Society knee score (KSKS) and preoperative Knee Society functional score (KSFS) were documented and preoperative knee extension and flexion angles were measured by using a 2-arm goniometer with the patients in the supine position. In our study, pre- and postoperative knee range of motion measurements were performed and compared by a single observer. Severity of knee osteoarthritis were classified based on the Ahlback grade system [23]. The visual analog scale (VAS) was used to assess the patient’s pain symptoms before UKA. Preoperative activity level, preoperative maximum flexion, preoperative maximum extension, varus deformity, and preoperative hip-knee-ankle axis (HKA) angle were evaluated.

RADIOLOGICAL MEASUREMENTS: Before and after surgery, all patients had anterior–posterior view X-ray examinations of the knee and both lower extremities performed. To measure the coronal hip-knee-ankle (HKA) angles before and after the surgery, we performed full-length standing radiography. The Oxford partial knee surgical technique operating manual was used to assess the alignment of femoral and tibial components [24]. We also recorded femoral component position relative to the femur with varus/valgus, flexion/extension, tibial component position relative to the tibia with varus/valgus, and anterior–posterior slope. Flexion and varus in the femoral component as well as varus and posterior slope of the tibial component were considered positive values.

SURGICAL TECHNIQUES: All patients underwent surgery performed by the same surgical team, and the surgeries were all conducted by a highly skilled and experienced surgeon. All operations were done with a minimally invasive approach. With the knee in moderate flexion, an incision was made medial to midline, extending from the proximal pole of the patella to just medial to the tibial tubercle. The tibial alignment was set to 90 degrees to the mechanical axis in the frontal plane and 7 degrees to the posterior slope in the sagittal plane during osteotomy. Achieving a 90-degree angle with the mechanical axis in the frontal plane was the goal for femoral alignment, with flexion ranging from 5 to 15 degrees in the sagittal plane. This can be achieved using either the Phase 3 or Microplasty system. After the osteotomy procedures, we conducted gap balancing and employed the implantation technique (Figure 1).

POSTOPERATIVE REHABILITATION:

All patients were treated with the same rehabilitation protocols. All patients were instructed by healthcare professionals to perform quadriceps exercises 3 times daily, with approximately 20 repetitions each time. Specifically, each patient would sit in long sitting posture with the back against the wall. For a period of 10 seconds, the patient would perform a maximal active isometric contraction of the quadriceps with the ankle in a dorsiflexed position. Twenty repetitions per set, 3 repetitions per day, and 5 repetitions per week were performed for the isometric contraction. Each exercise activity was completed under the supervision of healthcare professionals during the hospitalization of all patients. The patients received both oral and written instructions for home exercises upon discharge from the hospital. As part of their instructions, they were also advised to perform ankle pump exercises promptly. This advice aimed to reduce the risks associated with deep vein thrombosis (DVT) and pulmonary embolism (PE), as well as enhance the tracking of the patella. On the first day after surgery, patients were encouraged to start partial weight bearing and engage in knee exercises that involve both assisted range of motion (ROM) and active participation.

OUTCOME OF INTEREST:

Stiffness after UKA is defined as flexion <90° or a flexion contracture >10° (lack of extension).

STATISTICAL ANALYSIS:

The statistical analysis was carried out using Excel 2016 for Windows (Microsoft Corporation, Seattle, WA, USA) and SPSS 19.0 statistical software for Windows (IBM, Armonk, NY, USA). The factors associated with stiffness after UKA were identified using univariate analysis. Frequency is used to express categorical variables, while mean±SD is used to express continuous variables. The t test and chi-square test were used. A multivariate logistic regression model was built to identify the potential risk factors for stiffness after UKA. A stepwise regression method was used. A P value less than 0.05 was considered to be statistically significant.

Results

DEMOGRAPHIC AND GENERAL INFORMATION:

We finally included 590 knees in the study after UKA, comprising 296 male and 294 female patients with knee osteoarthritis. At the 1-year follow-up, the incidence of postoperative stiffness in UKA surgery was 10.17%. There were no differences in the demographic and baseline characteristics between the 2 groups, including the body mass index (25.18±3.22 vs 25.03±3.10, P=0.356), sex (50.17% vs 49.83%, P=0.752), smoking (81.36% vs 18.64%, P=0.485), hypertension (89.66% vs 10.34%, P=0.793), OA degree (32.2% vs 67.8%, P=0.370), diabetes(90.68% vs 9.32%, P=0.613), preoperative KSFS (55.4±13.2 vs 50.4±19.7, P=0.081), osteoporosis (0.18±1.37 vs 0.19±1.39, P=0.957), preoperative activity level (39.83% vs 60.17%, P=0.122), preoperative KSKS (45.4±10.0 vs 43.4±10.0, P=0.052), preoperative maximum extension (4.9±4.4 vs 4.7±3.5, P=0.106), and VAS score (3.92±2.45 vs 4.08±2.50, P=0.759). There were significant differences in age (39.83% vs 60.17%, P<0.001) and preoperative maximum flexion (132.0±6.4 vs 126.8±7.2, P<0.001) between the 2 groups. The patients’ baseline characteristics are summarized in Tables 1 and 2.

In terms of the radiological measurements, none of the following radiological characteristics differed between the 2 groups: preoperative HKA angle (169.9±4.3 vs 169.6±4.1, P=0.112), postoperative HKA angle (175.3±3.0 vs 175.6±3.2, P=0.763), femoral component varus/valgus angle (1.3±3.3 vs 0.8±4.0, P=0.530), femoral component flexion/extension angle (12.6±3.5 vs 12.8±7.2, P=0.875) and tibial component varus/valgus angle (2.0±1.3 vs 2.4±1.3, P=0.536). Varus deformity (70.34% vs 29.66%, P<0.001) and tibial component posterior slope angle (4.8±2.0 vs 4.6±2.0, P<0.001) were observed to have significant differences between 2 groups. Tables 2 and 3 give a summary of the radiological characteristics of the patients.

INDEPENDENT RISK FACTORS:

Multivariate logistic regression was used to identify 4 independent risk factors for stiffness after UKA. Age was risk factor for stiffness after UKA. The likelihood of becoming stiff after a UKA increased by 3.5% with each year of increasing age (95% CI, 1.022–1.048). The risk factors for stiffness after UKA were varus deformity (95% CI, 1.186–1.192) and tibial component posterior slope angle (95% CI, 0.550–0.870) in the radiological measurements. Stiffness after UKA was associated with preoperative maximum flexion as another risk factor. The likelihood of stiffness after UKA increased by 9.1% for every 1 degree decrease in preoperative maximum flexion (95% CI, 0.896–0.923). The independent risk factors for stiffness after UKA are summarized in Table 4.

Discussion

LIMITATIONS:

There are some limitations of the study. Firstly, it was a single-center study where all surgeries were performed by the same group of surgeons. However, the influence of surgeons’ surgical skills could not be evaluated. Similarly, the effect of the degree of postoperative rehabilitation could not be assessed. Second, some anatomical and geometric characteristics of the knee were measured on standard X-ray images. Therefore, the projection position might affect the measurement accuracy. Finally, we assessed final outcomes at 1 year postoperatively, which is a relatively short period for evaluating postoperative outcomes. However, as we mainly focused on postoperative knee stiffness, not on the service life of the prosthesis, we believe that 1 year is sufficient to assess patient knee function. Further prospective studies and basic experiments are needed to clarify the mechanisms involved in developing and progressing postoperative stiffness in UKA.

Conclusions

Four independent risk factors were identified in this study: age, varus deformity, preoperative maximum flexion, and tibial component posterior slope angle. Comprehending these risk factors might help surgeons prevent the occurrence of these postoperative knee stiffness in patients with unicompartmental knee arthroplasty. Based on our research, we recommend the following steps to avoid postoperative knee stiffness in patients with anteromedial knee osteoarthritis undergoing unicompartmental knee arthroplasty. First, it is vital to emphasize the significance of perioperative rehabilitation exercises, which can aid in improving knee joint flexibility and overall function. Second, for patients with maximum flexion reduction before the operation and significant preoperative varus deformities, we recommend early intervention whenever possible. Finally, a higher posterior tibial slope could also be advantageous in preventing postoperative knee stiffness.