Incidence of suboptimal femoral component sizing in Oxford Unicompartmental Knee Arthroplasty and comparisons for patient-reported outcomes with the optimal group: A retrospective cohort study in Taiwan

doi:10.21203/rs.3.rs-2582591/v1

Background: The Oxford Unicompartmental Knee Arthroplasty (OUKA) is a reliable surgical procedure for anteromedial osteoarthritis of the knee. However, in Taiwan, the incidence and patient-reported outcomes in suboptimal femoral component sizing remain unclear. Therefore, we conducted this study to clarify these issues.

Methods: We identified all patients receiving OUKA in a Taiwan medical center between May 1, 2018, and August 31, 2021, for this study. Age, sex, height, weight, body mass index, preoperative and postoperative Knee Society Score (KSS) functional activity score, and postoperative range of motion (ROM) and numeric rating scale (NRS) were included. Optimal and suboptimal groups were classified based on the posterior fit of the femoral component on postoperative lateral radiography. Outcome comparisons were performed between the two groups after following up.

Results: Ninety-two patients (111 knees) were included in the study, including 79 (71.2%) knees in the optimal group and 32 (28.8%) knees in the suboptimal group. The median age was 68 years and 67 years, and the percentage of the male sex was 17.7% and 37.5% in the optimal and suboptimal groups, respectively. Compared with the suboptimal group, there was a greater improvement in KSS functional activity score in the optimal group; however, the difference was not significant (40 vs. 34, p = 0.154). Nonsignificant differences were also found between the two groups in the postoperative KSS functional activity score and the postoperative NRS and ROM.

Conclusions: This study delineated the incidence and patient-reported outcomes in suboptimal femoral component sizing after OUKA in Taiwan, which provides an important reference for clinicians. Further studies, including more patients, are needed for validation.

femoral component sizing

Oxford Unicompartmental Knee Arthroplasty

outcome

posterior overhang

posterior underhang

Knee osteoarthritis (OA) is an age-related degenerative disease that could cause severe knee pain and leads to restricted activity and even disability[1]. The prevalence of knee OA increases with age[2, 3]. In individuals over 60 years, it is estimated that 13% of women and 10% of men have symptomatic knee OA[2, 3]. Its prevalence rapidly increases to 40% in people older than 70[2, 3]. In addition to its high prevalence, knee OA contributes to a huge medical expenditure[4]. A study in the United States showed that the average lifetime cost of a patient diagnosed with knee OA was about $140,300[4]. The high prevalence and cost of knee OA highlighted the need for effective treatment.

Anteromedial osteoarthritis (AMOA) of the knee, a specific aspect of knee OA, involves cartilage wear at the anterior side of the tibia plateau and leaves the posterior part intact[5]. This feature of AMOA enables diagnosing it from lateral radiographs, thereby making it suitable for treatment by unicompartmental arthroplasty[6]. Oxford Unicompartmental Knee Arthroplasty (OUKA) has proved to be a reliable surgical treatment for patients suffering from AMOA[6–8]. The phase 1 OUKA prosthesis was first developed in 1982[9] and then revised to phase 2 in 1987[10, 11]. Both phase 1 and phase 2 OUKA prostheses have only one size of the femoral component, which may not match patients’ unique knee geometries[12]. The phase 3 OUKA prosthesis was introduced in 1998 to improve the matching problem and has five different femoral component sizes[12]. In 2012, microplasty instrumentation was released[13]. The innovation of the phase 3 OUKA prosthesis leads to the development of various sizing methods[12]. The existing ways to predict and determine the size of the femoral component include the preoperative template[14, 15], height- and gender-based guidelines of different ethnicities[16, 17], an intraoperative sizing spoon[18], an intraoperative C-arm intensifier[19], and preoperative magnetic resonance imaging[20]. Despite the effort, posterior overhang or underhang of the femoral component is often found on the postoperative radiograph.

Improper implant size may be associated with complications, including bearing dislocation[21] and pain after phase 3 OUKA[22, 23]. In addition to implant size, differences in race and design of UKA (e.g., fixed bearing and mobile bearing) may also affect patient-reported outcomes[21–23]. However, the incidence of suboptimal femoral component sizing of phase 3 OUKA prosthesis and its effect on patient-reported outcomes remains unclear, particularly in Taiwan. Therefore, we conducted this study to clarify this issue.

2.1 Study design, setting, and participants

This study was conducted in the Chi Mei Medical Center (CMMC), a 1288-bed tertiary medical center in Taiwan[24]. For this study, we retrospectively identified 99 patients (122 knees) who received cemented OUKA in the CMMC between May 1, 2018, and August 31, 2021 (Figure 1). Patients lost to follow-up within 1 year or diagnosed with spontaneous osteonecrosis of the knee were excluded. In the final analysis, 92 patients (111 knees) were included. All surgeries were performed by one surgeon (CSC, the corresponding author).

2.2 Variables and data collection

An orthopedist collected the data retrospectively from electronic medical records, including a subgroup of AMOA (partial thickness cartilage loss [PTCL] and full thickness cartilage loss [FTCL]), age, sex, height, weight, body mass index (BMI), preoperative and postoperative Knee Society Score (KSS) functional activity score[25-27], postoperative range of motion (ROM), and postoperative numeric rating scale (NRS). The 2011 KSS functional activity score adopted in this study is the latest version published by the knee society in 2011. It is a well-validated scoring system and could provide more practical information on the patient’s functional recovery after the operation compared than the previous version[25-27]. The 2011 KSS consists of four separate subscales, including “objective knee score” (seven items: 100 points), patient satisfaction score (five items: 40 points), patient expectation score (three items: 15 points), and functional activity score (nineteen items: 100 points)[26]. Only the functional activity score of the 2011 KSS was included in this study to focus on the functional outcome of interest[26].

2.3 Surgery technique and criteria of the optimal and suboptimal groups

All surgeries were performed based on the OUKA surgical manual[28]. The size of the femoral component was determined using the sizing spoon intraoperatively to evaluate the proper ligament tension and the thickness of the articular cartilage loss[28]. During the postoperative follow-up, a lateral knee radiograph was performed to evaluate the position of the prosthesis[28]. The optimal cut limit was set at 2 mm[17]. If the cut edge of the femur was well aligned with the edge of the femur prosthesis or the distance between the bone edge and the implant was less than 2 mm, the knee was classified as the “optimal” group (Figure 2A)[17]. If the prosthesis exceeded the resected bone edge by over 2 mm, it was defined as a posterior overhang and classified as a “suboptimal group” (Figure 2B)[17].

In contrast, if the cutting edge of the bone showed protrusion more than 2 mm from the edge of the femur prosthesis, it was defined as a posterior underhang and classified as a “suboptimal group” (Figure 2C)[17]. The interpretation of the radiograph was performed by two orthopedic surgeons independently. If disagreement existed between two orthopedic surgeons, the operating surgeon made the final decision.

2.4 Outcome measurements

We compared patient-reported outcomes, including postoperative KSS functional activity score, improvement of KSS functional activity score, postoperative NRS, and postoperative ROM between the optimal group and the suboptimal group by following up.

2.5 Ethical statement

This study was conducted under the Declaration of Helsinki after the approval of the institutional review board of the CMMC. All the data in this study were anonymized. The patient’s informed consent was waived because the study was retrospective and observational, and the waiver did not affect the patient’s welfare.

2.6 Statistics

All statistical analyses were performed using SAS Version 9.4 for Windows software (SAS Institute, Cary, NC, USA). Categorical variables were compared using Pearson’s chi-square test. After the normality test, the Wilcoxon ranked-sum test was used for comparing the continuous data because they were not normally distributed. All reported p-values were 2-tailed, and the statistical significance was set at 0.05.

This study included 92 patients with 111 operated knees (Table 1). The mean follow-up time was 2.5 years (1.3–4.5 years). There were 79 knees (71.2%) in the optimal group and 32 (28.8%) in the suboptimal group. The suboptimal group included 20.7% of posterior overhang and 8.1% of posterior underhang. The percentage of male sex in the suboptimal group was higher than the optimal group (37.5% vs. 17.7%, p=0.026). The subgroups of AMOA were 27.9% for PTCL and 72.1% for FTCL. The median preoperative KSS functional activity score was 30 in the optimal group and 28 in the suboptimal group. The two groups had no significant differences in height, weight, BMI, and the preoperative KSS functional activity score.

Compared with the suboptimal group, the postoperative KSS functional activity score and the improvement of the KSS functional activity score were better in the optimal groups. However, these differences were not statistically significant (postoperative KSS functional activity score: 71 vs. 64.5, p = 0.403; improvement of KSS functional activity score: 40 vs. 34, p = 0.154) (Table 2). In the comparisons of the postoperative ROM and the postoperative NRS, there was no significant difference between the two groups.

The incidence of the suboptimal femoral component after OUKA in Taiwan is comparable with previous studies in other countries, which reported the percentage to be between 25.4% and 33.9% [17, 23]. Compared with previous studies, the percentage of posterior overhang is similar (20.7% vs. 17.7%–30.3%)[17, 23], and the percentage of posterior underhang is higher (8.1% vs. 3.6%–7.7%)[17, 23].

The possible reasons for the differences in incidence between studies are the differences in knee anatomy among different races[29, 30] and the surgeon’s judgment of appropriate implant size[31]. A study using computed tomography or magnetic resonance imaging showed that the knees of Chinese people are generally smaller than those of white people[32]. A systemic review showed that East Asians have smaller femoral anterior-posterior measurements than white people[12]. Even different Asian ethnicities have different-sized femurs[33]. A study showed that distal femur size in the Malay population is smaller than the Chinese but larger than Indians[33]. The high percentage of posterior overhang seemed reasonable since previous reports from Korea and India showed that even the smallest size of the femoral component, namely the extra-small, might be too large for Asian women[17, 34].

The higher male percentage in the suboptimal group than in the optimal group may be due to the surgeon’s choice. Most of the male patients in the suboptimal group had posterior overhang of the femoral component. The human knee is generally larger in male patients than in female patients[35]. Therefore, the surgeon might favor using a larger prosthesis for male patients when the result of the sizing spoon was equivocal, which contributed to the overhang.

Although the suboptimal group did not show significantly poorer patient-reported outcomes in this study, the posterior overhang of the femoral component still needs attention because it not only leads to the unsatisfactory radiographic presentation but also may increase the risk of a bearing dislocation[21, 36, 37]. Therefore, there is a need for an Asian-specific implant and an intraoperative accurate sizing technique for addressing anatomical variations to achieve a better component match.

The small sample size is a possible explanation for the nonsignificant difference of the postoperative KSS functional activity score and the improvement of the KSS functional activity score in this study. If we include more patients in the study, improving the KSS functional activity score in the optimal group may become more significant than the suboptimal group. Although the small sample size may be the reason, some studies report no significant difference in function scores between the optimal and suboptimal groups. A study in Germany including 59 patients with 61 knee joints who underwent OUKA showed no significant difference between the posterior overhang group and the nonoverhang group based on the American KSS[23]. Another two studies conducted in Korea also showed that the American KSS and the Western Ontario and the McMaster Universities Osteoarthritis Index scores did not differ between the overhang and nonoverhang groups[34, 38]. We used the KSS functional activity score as the outcome measurement in this study because it has a wider range of activities than other scoring systems and blanks for the patients to fill in their discretionary knee activities[25]. Surgeons could better evaluate whether the patient’s functional recovery met their daily needs through this scoring system[25].

Other factors, including the optimal implantation of the tibial component, may also explain the nonsignificant difference in the postoperative ROM and the postoperative NRS between the optimal and suboptimal groups. Even if the femur component is undersized, the knee flexion may not be affected if the tibia component is well implanted, and the gap balance is good. In addition, the postoperative ROM in OUKA is generally good[39, 40]. Therefore, the appropriate implantation may not be a major outcome factor. The postoperative NRS in most patients was 0 or 1, and only 5 patients in our study had a postoperative NRS of more than 5. Therefore, the appropriate implantation may become a minor factor for postoperative NRS.

The major strength of this study is that it is the first to investigate the incidence of suboptimal femoral component sizing in OUKA and its association with patient-reported outcomes in Taiwan. In addition, we used the KSS functional activity score as the outcome measurement. This questionnaire includes a broader range of daily activities and could give the surgeons a more detailed view of the patient’s functional recovery after surgery. There are some limitations as follows. First, some factors were not collected completely since the study was retrospective, including preoperative ROM and NRS. Therefore, we could not compare the difference in ROM and NRS between preoperative and postoperative periods. Second, interobserver variations on radiograph reading and the amount of posterior overhang or underhang may exist. Third, the case number is relatively small, which may not reveal the difference between the optimal and suboptimal groups. Further studies including those with a future design, more patients, and more precise methods for measuring the level of posterior overhang or underhang (e.g., computed tomography) are needed.

This study showed that the incidence of suboptimal femoral component sizing after OUKA was 28.8% in Taiwan. There was no significant difference in the postoperative KSS functional activity score, improvement of the KSS functional activity score, postoperative NRS, and postoperative ROM between the optimal group and the suboptimal group. Although the difference was not statistically significant, we observed a better improvement of the KSS functional activity score in the optimal group than in the suboptimal group. The result could fill the gap for Taiwan data about this issue and provide an important reference for clinical practice. Future studies, including more patients, are needed to validate the finding.

OA: osteoarthritis

AMOA: Anteromedial osteoarthritis

OUKA: Oxford Unicompartmental Knee Arthroplasty

CMMC: Chi Mei Medical Center

PTCL: partial thickness cartilage loss

FTCL: full thickness cartilage loss

BMI: body mass index

KSS: Knee Society Score

ROM: range of motion

NRS: numeric rating scale

Ethics approval and consent to participate

This study and all experimental protocols were conducted under the Declaration of Helsinki after the approval of the institutional review board of the Chi Mei Medical Center (IRB number:11110-009). All the data in this study were anonymized. The patients’ informed consent was waived by the institutional review board of the Chi Mei Medical Center because the study was retrospective and observational, and the waiver does not affect patients’ welfare.

Consent for publication

Not applicable.

Availability of data and material

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no conflicts of interest.

Funding

Nil.

Authors’ contributions

WSC, KTC, CC Huang, CSC designed and conceived this study. CHH performed the statistical analysis. TMW assisted in the implementation. All the authors wrote, read, and approved the final manuscript.

Acknowledgments

We thank Enago for their English language editing services.

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Table 1. Comparison of clinical characteristics between the optimal and the suboptimal group in all the AMOA knees undergoing OUKA

Clinical characteristics	Overall	Optimal group	Suboptimal group	p-value*
Clinical characteristics	n = 111 (100)	n = 79 (71.2)	n = 32 (28.8)	p-value*
Age (years)	67.0 (62.0–71.0)	68.0 (62.0–71.0)	67.0 (62.0–72.0)	0.997
Sex				0.026
Female	85 (76.6)	65 (82.3)	20 (62.5)
Male	26 (23.4)	14 (17.7)	12 (37.5)
Subgroup of AMOA				0.620
PTCL	31 (27.9)	21 (26.6)	10 (31.3)
FTCL	80 (72.1)	58 (73.4)	22 (68.8)
Height (cm)	156.0 (152.0–160.0)	156.0 (152.0–160.0)	157.0 (150.0–164.0)	0.600
Weight (kg)	66.0 (60.0–77.3)	67.0 (60.0–78.0)	65.0 (60.85–75.0)	0.747
BMI (kg/m²)	27.6 (25.0–29.8)	27.7 (24.7–29.8)	25.6 (25.3–29.9)	0.790
Preoperative KSS functional activity score	29 (25–33)	30 (25–32)	28 (25.5–38.5)	0.7401

Data were presented as n (%) or median (Q1–Q3). *Categorical variables are analyzed by Pearson’s chi-square test and continuous variables are analyzed by the Wilcoxon ranked-sum test. AMOA, anteromedial osteoarthritis of the knee; OUKA, Oxford Unicompartmental Knee Arthroplasty; PTCL, partial thickness cartilage loss; FTCL, full thickness cartilage loss; BMI, body mass index; KSS, Knee Society Score.

Table 2. Comparison of patient-reported outcomes between the optimal group and the suboptimal group in all the OA knees undergoing OUKA

Outcome measurement	Overall	Optimal group	Suboptimal group	p-value*
Outcome measurement	n = 111 (100)	n = 79 (71.2)	n = 32 (28.8)	p-value*
Postoperative KSS functional activity score	70 (56–76)	71 (60–76)	64.5 (55.5–76.5)	0.403
Improvement of KSS functional activity score	39 (25–46)	40 (31–46)	34 (20.5–44.5)	0.154
Postoperative ROM	120 (115–125)	120 (115–125)	120 (120–125)	0.817
Postoperative NRS	1 (0–1)	1 (0–1)	1 (0–5)	0.817

Data were presented as n (%) or median (Q1–Q3). *Wilcoxon ranked-sum test. OA, osteoarthritis; OUKA, Oxford Unicompartmental Knee Arthroplasty; KSS, Knee Society Score; ROM, range of motion; NRS, numeric rating scale.

No competing interests reported.

Incidence of suboptimal femoral component sizing in Oxford Unicompartmental Knee Arthroplasty and comparisons for patient-reported outcomes with the optimal group: A retrospective cohort study in Taiwan

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Abstract

Figures

1. Introduction

2. Materials And Methods

3. Results

4. Discussion

5. Conclusions

Abbreviations

Declarations

References

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Additional Declarations

Status:

Version 1