Study design
This retrospective study was conducted at the Department of Orthopedics in the Second Affiliated Hospital of Shanxi Medical University. The protocol for the study has been approved by the Ethics Committee of the Second Affiliated Hospital of Shanxi Medical University and performed in a manner consistent with the Helsinki Declaration [15]. Written informed consent to participate was obtained from all patients.
Inclusion and exclusion criteria
Patients eligible for inclusion in this study were individuals who were: (1) diagnosed with knee osteoarthritis and underwent unilateral OUKA at a single hospital between February 2016 and June 2019; (2) implanted with a third-generation Oxford mobile platform prosthesis; (3) exhibited a preoperative CTFS ≤ 1 cm; and (4) had undergone follow-up for ≥ 2 years.
Patients were excluded from this study if they: (1) had a history of prior knee joint surgery; (2) exhibited anterior cruciate ligament (ACL) dysfunction that was detected intraoperatively and underwent simultaneous ACL reconstruction; or (3) were lost to follow-up or had incomplete follow-up data.
In total, 221 patients underwent OUKA treatment for medial single-compartment osteoarthritis over this study period. Of these patients, 183 (62 male, 121 female) met with study inclusion criteria, with an average age of 66.1 years. Clinical data (demographic and functional evaluation data) and radiological parameters (CTFS value and mFTA) for all patients were recorded.
Surgical methods
The same surgeon treated all patients, all of whom were implanted with an Oxford unicondylar prosthesis. Following anesthetization, patients were placed in the supine position. This limb was placed in a unicondylar leg frame such that the hip joint was flexed 30° with slight abduction while the lower leg drooped naturally, allowing the passive range of motion (ROM) of the knee to reach 110°. A tourniquet was used to disrupt blood flow to the affected limbs.
This surgical procedure was conducted via a medial patellar approach, with the knee joint bent 90°. A 6–7 cm incision was made at the medial edge of the patella extending from above the joint capsule to the tibial tubercle at the distal end. The skin, subcutaneous tissue, and joint capsule were cut in sequence. An Oxford bone spoon of proper thickness was then selected based upon the medial joint space and articular cartilage wear. A vertical osteotomy was then performed on the intercondylar crest of the tibia close to the inner side of the ACL and directed towards the anterior superior iliac spine. After carefully protecting the medial collateral ligament with a Z-shaped retractor, a horizontal osteotomy was then performed, and the osteotomy block was removed and compared with a corresponding test model to determine the sizing of the tibial prosthesis. After the knee joint was flexed 45°, the femoral condyle was fully exposed and a femoral cavity positioning rod was inserted in a retrograde manner, after which the femoral drilling guide was installed. Drill holes along the guide were made with 4 mm and 6 mm electric drills. The femoral posterior condyle osteotomy plate was placed in the femoral condyle positioning hole to adhere to the femoral condyle osteotomy, after which the grinding bolt was placed in the femoral positioning hole, and the distal femur was ground with the grinding drill.
The unicondylar prosthesis was then installed, the flexion–extension gap was repeatedly tested to ensure there was no impact or dislocation. The joint capsule was then washed repeatedly, the joint capsule, subcutaneous tissue, and skin were sutured under sufficient hemostasis, and sterile dressings and elastic bandages were applied to compress the wound.
Postoperative treatment
Immediately after patients had recovered from anesthetization, ankle pumping and straight leg lifting training were initiated. On day 1 postoperatively, some patients were able to walk with the aid of a walker. Oral anticoagulants were routinely administered to prevent complications such as deep venous thrombosis of the lower limbs. After discharge, patients were instructed to exercise actively, such that the ROM of the knee joint generally extended to 0–100° within 1 week. After 2 weeks, stitches were removed and full weight-bearing walking exercises were initiated.
Result evaluations
The evaluation process was simultaneously performed by three observers who assessed clinical and imaging parameters. Each relevant measurement or assessment value was performed twice by each of these observers, with the average value then being taken for analysis. The intraclass correlation coefficient (ICC) was used to evaluate the interobserver reliability and retest reliability. If the reliability coefficient was less than 0.4, the reliability was considered poor, whereas if it was greater than 0.75, the reliability was considered to be high.
Evaluation of clinical function
All clinical information for these patients was collected by a research assistant using a predesigned case report form at 6 weeks, 3 months, 6 months, 12 months, and every year thereafter postoperatively. Outpatient follow-up was conducted using paper questionnaires or via telephone. Primary clinical measurement indices for these patients included the OKS and HSS scores, the former of which has been confirmed to be a reliable evaluation tool following OUKA surgery and the latter of which offers insight regarding changes in the status of the knee joint in knee osteoarthritis patients before and after surgical treatment. Clinical outcomes were described in terms of the percentage of maximum possible improvement (PMPI), rather than by using a predetermined value of the minimum clinically important difference (MCID) as this was considered to be limited by the ceiling effect. A cut-off value of 30% was used to differentiate between the number of patients who were satisfied (PMPI > 30%) and dissatisfied (PMPI < 30%) with their surgical procedure as determined based upon HSS score results. This cut-off value has previously been used in the joint replacement literature to indicate significant and functional improvement [16, 17]. The basic principle of this method is that it sets a higher standard than does MCID, increasing the number of patients with unsatisfactory results and allowing comparisons between these patients and those with satisfactory results. During follow-up, patients graded their operative satisfaction as follows: disappointed, dissatisfied, neutral, satisfied, or very satisfied. ROM values and complication rates in each patient group were additionally recorded, and VAS scores were used to rate the degree of knee pain before and after surgery.
OKS scores
OKS scores consist of 12 items, each of which was initially scored from 1–5 points, with one corresponding to the slightest and five corresponding to the most serious outcome for that item. Total scores range from 12 points (best) to 60 points (worst). Many scholars eventually suggested that these scores be adjusted to range from 0–4, with a maximum possible score of 48 [18]. Accordingly, the latest revised OKS scoring system rates each item from 0–4, with total possible scores ranging from 0 (worst) to 48 (best).
HSS scores
The HSS knee score is a scoring system proposed in 1976 by the American Second Hospital of Special Surgery to evaluate knee joints before and after surgical procedures. This scale assesses six key aspects: pain, function, ROM, muscle strength, knee flexion deformity, and knee instability. The total possible HSS score is 100 points (pain: 30 points; function: 22 points; ROM: 18 points; muscle strength: 10 points; knee flexion deformity: 10 points; knee instability: 10 points). The results are divided into four grades: excellent (> 85 points), good (70–84 points), moderate (60–69 points), and poor (< 59 points) [19].
VAS scores
A 10 cm horizontal line was drawn on a sheet of paper, with a 0 at one end of the line indicating no pain, and a 10 at the other end indicating severe pain. Patients were directed to indicate the region on this scale that matched their degree of pain. The distance from 0 to the marked position was then measured to determine the VAS score for a given patient [20].
Imaging parameters
In radiographic images, the same source to image distance (SID) before and after operation was measured, and the SID value was set to 80 cm. Subluxation of the tibia relative to the femur was measured on preoperative anterior and posterior X-ray images of the knee joint in a weight-bearing position. Based on the degree of subluxation, patients were separated into three groups: normal, mild subluxation, and severe subluxation groups. At their most recent follow-up, anterior and posterior X-ray examinations of these patients were again performed to measure the degree of correction of such subluxation by the OUKA procedure. In addition, mFTA was measured using the full-length films of both lower limbs to judge lower limb alignment before and after surgery.
CTFS measurement
Many approaches to measuring CTFS have been described to date. As the diseased knee joint often lacks clear anatomical landmarks owing to osteophyte hyperplasia, hypertrophy, or other defects, such measurements can be challenging. For this study, CTFS measurements were made as per a protocol published by Nam et al. [14], which avoids the influence of osteophytes while also reducing the influence of the slight rotation of X-ray film (Fig. 1).
Complications
Complications such as incision problems, neurovascular injury, periprosthetic infection, deep vein thrombosis (DVT), pulmonary embolism (PE), knee joint stiffness, prosthesis loosening, gasket dislocation, and osteoarthritis progression in the lateral compartment were analyzed for all patients.
Statistical analysis
SPSS 26.0 (IBM, IL, USA) was used for all statistical analyses. Quantitative data are given as means ± standard deviation, and were compared using paired t-tests, independent-sample t-tests, and one-way ANOVAs with Bonferroni correction, as appropriate. Categorical data were given as absolute frequencies (n), and were compared via Pearson’s Chi-squared test with a two-sided α-value of 0.05, and P < 0.05 as the threshold for significance.