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Abstract

Background:

Mucoid degeneration (MD) of the anterior cruciate ligament (ACL) is a recognized pathology marked by collagen fiber degradation and infiltration of a mucoid substance, which may be associated with the subsequent development of knee osteoarthritis.

Purpose:

To investigate the association between progressive MD-ACL and intermediate-term outcomes following high tibial osteotomy (HTO) for medial compartment knee osteoarthritis.

Study Design:

Cohort study; Level of evidence, 3.

Methods:

The authors identified 322 patients who underwent medial opening-wedge HTO for medial compartment osteoarthritis with an intact ACL between 2015 and 2022. All patients underwent second-look arthroscopy approximately 2 years after the initial HTO, with a mean follow-up of 72.5 months (range, 27.0-116.0) from the initial HTO to the latest evaluation. Patients with MD-ACL observed during second-look arthroscopy were classified as the MD-ACL group, while those with a normal ACL served as the control group. Propensity score matching was conducted by age, sex, body mass index, and the interval between HTO and second-look arthroscopy, resulting in 43 matched patients in each group. Clinical outcomes were evaluated with the International Knee Documentation Committee (IKDC) score and Knee injury and Osteoarthritis Outcome Score (KOOS).

Results:

Both groups showed significant improvements in clinical outcomes at second-look arthroscopy as compared with baseline (all P < .001). However, at second-look arthroscopy, the MD-ACL group had significantly worse IKDC scores (mean ± SD, 61.0 ± 13.9 vs 68.8 ± 14.4; P = .020), KOOS pain (70.1 ± 19.1 vs 78.6 ± 15.0; P = .023), KOOS symptoms (67.3 ± 19.5 vs 76.1 ± 13.6; P = .022), KOOS activities of daily living (70.7 ± 18.9 vs 80.4 ± 15.5; P = .011), and range of motion (128.8°± 8.9° vs 136.1°± 7.8°; P < .001) as compared with the control group. At the latest follow-up, outcomes in the control group remained stable without significant decline (P = .568), while the MD-ACL group demonstrated a clear tendency toward deterioration, including a significant decline in IKDC scores (61.0 ± 13.9 at second-look arthroscopy to 57.4 ± 12.3 at the latest follow-up; P = .025). The survival rate at a mean follow-up of 72.5 months was 97.6%, with no significant difference between groups (P = .400, log-rank test). Multivariate regression analysis identified smaller intercondylar notch geometry (P = .003), increased postoperative medial proximal tibial angle (P = .031), and larger changes in posterior tibial slope (P = .004) as significant predictors of MD-ACL development.

Conclusion:

The development of MD-ACL was associated with poorer clinical outcomes after HTO over an intermediate follow-up period. Smaller intercondylar notch geometry, increased medial proximal tibial angle, and larger changes in posterior tibial slope were significant predictors of MD-ACL development. These findings highlight the importance of careful preoperative planning and meticulous attention to surgical techniques that avoid excessive increases in medial proximal tibial angle and PTS, thereby improving HTO outcomes.

Keywords

anterior cruciate ligament mucoid degeneration high tibial osteotomy osteoarthritis intermediate-term results

High tibial osteotomy (HTO) is an established surgical intervention for managing medial compartment osteoarthritis of the knee in patients with varus deformity.^37,41 Numerous clinical studies have demonstrated the efficacy of HTO in significantly improving clinical outcomes.^{7,16,19,27,48} However, the long-term durability of HTO outcomes is variable, with several risk factors implicated in the progressive decline of outcomes or reduced procedure survival over time.^{2,13,25,31,52,54}

Degenerative changes in the anterior cruciate ligament (ACL) can compromise knee function and proprioception, leading to altered joint kinematics.^47,49 Among these degenerative conditions, mucoid degeneration of the ACL (MD-ACL) is a well-recognized pathology. MD-ACL is characterized by the degradation of collagen fibers and the infiltration of a mucoid-like substance, which results in chronic knee discomfort and restricted knee range of motion.^{20,30,34,36,38} Current evidence suggests that MD-ACL may be associated with the subsequent development of knee osteoarthritis and represents a component of the broader degenerative process of the knee joint.^20,32 Given its pathophysiologic implications, MD-ACL may influence outcomes after HTO.

Several clinical investigations have explored the relationship between postoperative outcomes after HTO and various ACL abnormalities, including partial and complete tears as well as MD, as observed on magnetic resonance imaging (MRI).^28,39 These studies have reported unfavorable effects on the ACL after HTO, raising concerns regarding progressive ACL deterioration. However, there is limited research examining the association between the development of MD-ACL and intermediate-term outcomes after HTO.

The objective of this study was (1) to evaluate the prevalence of postoperative MD-ACL in patients who underwent HTO, as assessed by MRI and second-look arthroscopy, and (2) to determine the association between MD-ACL and postoperative clinical outcomes over an intermediate follow-up. We hypothesized that the development of MD-ACL after HTO would be significantly associated with worse clinical outcomes over time.

Methods

Patients

This study was approved by the institutional review board of our institution, and written informed consent was obtained from all patients who agreed to undergo second-look arthroscopy during plate removal. We retrospectively identified all patients who underwent HTO between January 2015 and September 2022 at a single center. Preoperatively, all patients were routinely recommended to undergo second-look arthroscopic surgery, with the procedure's purpose explained as the assessment of intra-articular status and determination of the need for additional arthroscopic procedures at least 1 year after the initial surgery. The indication for HTO was isolated medial compartment osteoarthritis (Kellgren-Lawrence grade ≥2) with varus malalignment in patients ≤65 years old without ligament instability. HTO was contraindicated in patients presenting with symptomatic lateral or patellofemoral compartment osteoarthritis.

Patients in this study had medial compartment knee osteoarthritis and an intact ACL confirmed by MRI and arthroscopic evaluation, followed by second-look arthroscopy performed at a minimum of 2 years after the initial surgery. Patients who declined second-look arthroscopy, precluding postoperative evaluation, were excluded. Additional exclusion criteria were as follows: evidence of ACL injuries on postoperative MRI or second-look arthroscopy, knee range of motion <120° or flexion contracture >15°, patellofemoral or lateral compartment osteoarthritis, previous knee surgery, and inflammatory arthritis or traumatic osteoarthritis.

Of the 322 patients (322 knees) initially evaluated, 43 were identified as having MD-ACL based on postoperative MRI and second-look arthroscopy performed during plate removal. To ensure inclusion of only new-onset MD-ACL cases, patients with preoperative evidence of MD-ACL on MRI or first-look arthroscopy at the time of HTO were excluded from the study cohort. The diagnosis of MD-ACL was established per the following criteria: absence of ACL tears, mucoid hypertrophy of the ACL characterized by the “celery stalk” appearance on MRI,⁴³ and yellowish degenerative lesions identified during probing and arthroscopy (Figure 1).^21,34,35 Each patient in the MD-ACL group was matched with a control patient who had a normal ACL on MRI and second-look arthroscopy at the time of plate removal. Propensity score matching was conducted to ensure comparability between groups. The propensity score was calculated by logistic regression analysis based on baseline variables including age, sex, body mass index, and the interval between HTO and second-look arthroscopy. One-to-one matching was performed with a variable-ratio, parallel, and balanced nearest-neighbor method with a caliper width of 0.2 standard deviations of the propensity score to limit the allowable distance between matched pairs. The balance of covariates between groups was assessed by calculating standardized mean differences.

Figure 1.

(A) Follow-up T2-weighted magnetic resonance imaging scan shows mucoid hypertrophy of the ACL with a distinctive “celery stalk” appearance. (B) Second-look arthroscopic image shows mucoid degeneration of the ACL. The hypertrophied ACL fills the entire notch, the synovial membrane is absent, but the fibers remain continuous. ACL, anterior cruciate ligament.

Surgical Technique and Postoperative Rehabilitation

All surgical procedures were conducted by 2 experienced orthopaedic surgeons (J.K.S., E.K.S.). Following arthroscopic assessment, a surgical incision was made on the superomedial aspect of the tibia. Under fluoroscopic guidance, 2 guide wires were inserted 35 mm distal to the joint line, originating from the metaphyseal-diaphyseal junction and directed toward the fibular tip. Biplanar medial opening-wedge HTO was then performed with an oscillating saw. The first oblique osteotomy plane was created 1 cm distal to the tibial tubercle, and the second was made along the guide wire trajectory, forming an angle of approximately 100° to 110° between the osteotomy lines. The osteotomy gap was gradually expanded to the predetermined wedge length, with intraoperative fluoroscopic confirmation of the targeted mechanical axis. Fixation was accomplished with a locking plate and screws. To promote osseous union, particularly when the gap width exceeded 10 mm, allograft cancellous bone chips were implanted.

Postoperative rehabilitation commenced on the first postoperative day with active and passive range of motion exercises. Partial weightbearing ambulation was permitted for the initial 6 weeks, facilitated by the use of a hinged knee brace. Progressive weightbearing was introduced thereafter. Approximately 2 years after the primary surgery, second-look arthroscopy was performed in conjunction with plate removal. Before this procedure, union at the osteotomy site was confirmed via radiographic evaluation. Simultaneous arthroscopic assessment during plate removal was conducted to evaluate ACL status.

Clinical Evaluation

Patient-reported outcome measures were assessed preoperatively and at multiple postoperative time points: 1, 3, 6, and 12 months after surgery and annually thereafter. The primary clinical outcome measure was the International Knee Documentation Committee (IKDC) questionnaire.²³ Secondary clinical outcome measures included the Knee injury and Osteoarthritis Outcome Score (KOOS)⁴⁶ and range of motion. All clinical outcomes were independently evaluated by 2 observers (Y.Z. and S.J.K.) who were blinded to the study's objectives and had not participated in the surgical procedures.

Radiographic Evaluation

Standardized anteroposterior, lateral, and Merchant views of the knee, as well as whole lower extremity weightbearing radiographs, were obtained preoperatively and during each follow-up visit. Radiographic evaluations included measurements of the hip-knee-ankle angle, posterior tibial slope (PTS), medial proximal tibial angle (MPTA), and joint line convergence angle (JLCA). The mechanical hip-knee-ankle angle was measured as the angle formed between the mechanical axis of the femur and the mechanical axis of the tibia, with positive values indicating varus alignment.^45,55 The MPTA was defined as the medial angle between the mechanical axis of the tibia and the joint line of the proximal tibia. The JLCA was defined as the angle between lines tangent to the femoral condyles and the tibial plateau, with the lateral opening designated as a positive value (Figure 2). The PTS was quantified as the angle between a line perpendicular to the proximal tibial anatomic axis and a line tangential to the medial tibial plateau (Figure 3).⁵¹ Osteoarthritis severity was assessed by the Kellgren-Lawrence grading system based on whole lower extremity weightbearing radiographs.²⁶

Figure 2.

Measurement of the HKA angle, MPTA, and JLCA on lower extremity weightbearing radiographs. (A) The HKA angle was determined by the intersection of the femoral and tibial mechanical axis lines. (B) The MPTA was defined as the medial angle between the tibial mechanical axis and the joint line of the proximal tibia. (C) The JLCA was defined as the angle between lines tangent to the femoral condyles and the tibial plateau. HKA, hip-knee-ankle; JLCA, joint line convergence angle; MPTA, medial proximal tibial angle.

Figure 3.

Measurement of PTS on a lateral radiograph. The PTS is defined as the angle between a line perpendicular to the proximal tibial anatomic axis and a line tangential to the medial tibial plateau. The proximal anatomic axis was defined as the line connecting the midcortical diameters of the tibia at 5 and 15 cm distal to the knee joint. PTS, posterior tibial slope.

MRI Assessment and Arthroscopic Evaluation

All patients in both groups underwent preoperative knee MRI and follow-up imaging at the time of second-look arthroscopy during plate removal. The morphologic status of the ACLs on MRI was assessed by 2 experienced musculoskeletal radiologists using a previously established staging system.^5,17,22 MD-ACL on MRI was characterized by an ill-defined, thickened ligament displaying a distinctive “celery stalk” appearance and increased signal intensity across all imaging sequences, including T2-, T1-, and proton density–weighted images.⁴³ Additionally, the notch width index, calculated as the ratio of the intercondylar notch width to the transcondylar width, was measured at the level of the popliteal groove on axial MRI views, as previously described (Figure 4).⁹

Figure 4.

Measurement of NWI on magnetic resonance imaging. The NWI is defined as the ratio of the intercondylar notch width (dashed white line) to the transcondylar width (solid white line). NWI, notch width index.

Macroscopic ACL status was assessed via index arthroscopy at the time of the initial HTO and second-look arthroscopy.^1,20 MD-ACL on arthroscopy was confirmed by observing a hypertrophied ACL with lateral compartment expansion and the presence of yellowish degenerative lesions within ACL fibers.^30,34 The medial femoral condyle articular cartilage was also evaluated during index arthroscopy according to the International Cartilage Repair Society grading system.¹⁰ All radiological and arthroscopic findings were independently evaluated by 2 board-certified orthopaedic surgeons (Y.Z. and S.J.K.). To evaluate intra- and interobserver reliability, assessments were repeated after a 4-week interval. For arthroscopic evaluations, reassessments were performed with archived video recordings of the procedures. The inter- and intraobserver measurement reliabilities were assessed by the intraclass correlation coefficient.

Predictive Factors Associated With the Development of MD-ACL

To assess predisposing factors associated with the development of MD-ACL, we examined the relationship between progressive MD-ACL and various variables, including age, sex, body mass index, time between HTO and second-look arthroscopy, and perioperative radiologic and arthroscopic findings.

Statistical Analysis

A post hoc power analysis showed that the sample sizes of the MD-ACL group (n = 43) and control group (n = 43) provided >80% statistical power to detect differences in the primary outcome, the IKDC score. For normally distributed variables, paired and independent t tests were performed to analyze differences. Nonnormally distributed variables were analyzed by the Wilcoxon signed rank test and the Mann-Whitney U test. Categorical variables were compared by chi-square test. Multivariable logistic regression analysis was performed to identify predictors associated with the occurrence of MD-ACL after HTO and to determine whether MD-ACL development influenced long-term joint preservation. Additionally, we evaluated whether the presence of MD-ACL was associated with an increased risk of clinical failure after HTO, defined as conversion to total knee arthroplasty. Kaplan-Meier survival analysis was performed, considering conversion to total knee arthroplasty as the endpoint. Log-rank tests were used to evaluate significant differences in survivorship curves based on the development of MD-ACL after HTO. Statistical significance was set at P < .05 for all analyses.

Results

Among the 322 eligible patients evaluated before propensity score matching, 45 (13.9%) demonstrated postoperative development of MD-ACL changes, as observed during second-look arthroscopy. The propensity score matching analysis generated 43 matched pairs of patients, achieving balance across covariates (standardized mean difference <0.2 for all variables). There were no statistically significant differences in the matching variables between the groups (P > .05; Table 1), confirming successful matching. The final matched cohort included 86 patients: 43 in the MD-ACL group and 43 in the control group. The mean follow-up was 72.5 months (mean ± SD, 73.0 ± 29.7 in the MD-ACL group and 72.1 ± 24.6 in the control group), with no statistically significant difference between groups (P = .874) (Figure 5). All radiologic and arthroscopic findings demonstrated high reliability, with intraclass correlation coefficients ranging from 0.89 to 0.95 for radiologic measurements and 0.90 to 0.97 for arthroscopic assessments, indicating good to excellent agreement (intraclass correlation coefficient >0.75).

Table 1

Baseline Characteristics Before and After Propensity Score Matching ^a

	No. or Mean ± SD
Variable	MD-ACL	Control	P Value ^b	SMD
Before matching
Patients	45	277
Age, y	57.7 ± 5.3	56.4 ± 6.0	.194	0.232
Sex			.185	0.295
Male	6	65
Female	39	212
Body mass index, kg/m²	26.8 ± 2.8	27.1 ± 3.9	.545	0.104
Interval between HTO and second-look arthroscopy, mo	24.6 ± 8.5	20.5 ± 11.4	.007	0.472
After matching
Patients	43	43
Age	57.3 ± 5.2	57.4 ± 5.7	.953	0.013
Sex			.999	0.068
Male	6	7
Female	37	36
Body mass index, kg/m²	26.8 ± 2.8	26.5 ± 3.8	.719	0.094
Interval between HTO and second-look arthroscopy, mo	24.0 ± 7.8	24.1 ± 13.5	.897	0.036

Bold indicates a statistically significant difference (P < .05). HTO, high tibial osteotomy; MD-ACL, mucoid degeneration of the anterior cruciate ligament; SMD, standardized mean difference.

An independent t test was used to analyze the differences in age, body mass index, and time interval between HTO and second-look arthroscopy. A chi-square test was used to analyze the differences in sex.

Figure 5.

Flowchart depicts the patient enrollment process in the study. ACL, anterior cruciate ligament; HTO, high tibial osteotomy.

Clinical Outcomes

As shown in Table 2, both groups exhibited significant improvements in all clinical outcome measures at the second-look arthroscopic evaluation as compared with baseline (all P < .001). However, the MD-ACL group demonstrated significantly worse IKDC scores, KOOS subscales (pain, symptoms, activities of daily living), and range of motion as compared with the control group at the second-look arthroscopy (P < .05). At the latest follow-up, outcomes in the control group remained stable without significant decline (P = .568), whereas the MD-ACL group showed a clear trend toward deterioration, including a significant decline in IKDC scores (P = .025). While the control group sustained improvements from preoperative levels, the MD-ACL group experienced progressive deterioration in clinical scores over time.

Table 2

Clinical Outcomes Between the MC-ACL and Control Groups ^a

	MD-ACL (n = 43)	Control (n = 43)	P Value ^b
Range of motion
Preoperative	134.0 ± 9.0	134.6 ± 10.1	.779
Second-look arthroscopic surgery	128.8 ± 8.9	136.1 ± 7.8	<.001
Latest follow-up	131.3 ± 12.0	137.2 ± 6.0	.006
P value ^c	<.001	<.001
P value ^d	.217	.333
IKDC
Preoperative	38.7 ± 10.8	39.8 ± 11.3	.637
Second-look arthroscopic surgery	61.0 ± 13.9	68.8 ± 14.4	.020
Latest follow-up	57.4 ± 12.3	69.1 ± 11.4	<.001
P value ^c	<.001	<.001
P value ^d	.025	.568
KOOS pain
Preoperative	47.3 ± 13.4	48.1 ± 13.0	.782
Second-look arthroscopic surgery	70.1 ± 19.1	78.6 ± 15.0	.023
Latest follow-up	67.8 ± 17.0	79.0 ± 10.3	<.001
P value ^c	<.001	<.001
P value ^d	.363	.875
KOOS symptoms
Preoperative	49.0 ± 15.9	50.4 ± 13.5	.616
Second-look arthroscopic surgery	67.3 ± 19.5	76.1 ± 13.6	.022
Latest follow-up	65.1 ± 13.8	77.6 ± 11.5	<.001
P value ^c	<.001	<.001
P value ^d	.433	.386
KOOS activities of daily living
Preoperative	49.6 ± 13.1	50.8 ± 12.9	.651
Second-look arthroscopic surgery	70.7 ± 18.9	80.4 ± 15.5	.011
Latest follow-up	69.1 ± 12.8	81.1 ± 8.0	<.001
P value ^c	<.001	<.001
P value ^d	.535	.736
KOOS sports and recreation
Preoperative	29.7 ± 12.6	29.8 ± 18.4	.967
Second-look arthroscopic surgery	34.6 ± 19.8	41.6 ± 17.0	.097
Latest follow-up	35.1 ± 19.6	43.4 ± 19.8	.054
P value ^c	<.001	<.001
P value ^d	.868	.473
KOOS quality of life
Preoperative	28.5 ± 15.1	29.3 ± 18.7	.815
Second-look arthroscopic surgery	51.1 ± 19.8	52.6 ± 19.7	.774
Latest follow-up	52.5 ± 19.1	55.5 ± 15.1	.465
P value ^c	<.001	<.001
P value ^d	.701	.589

Values are presented as mean ± SD. Bold indicates a statistically significant difference (P < .05). IKDC, International Knee Documentation Committee; KOOS, Knee injury and Osteoarthritis Outcome Score; MD-ACL, mucoid degeneration of the anterior cruciate ligament.

Mann-Whitney U test for analysis of differences.

Wilcoxon signed rank test for comparison of clinical outcomes between preoperative and second-look arthroscopic surgery time points.

Wilcoxon signed rank test for comparison of clinical outcomes between second-look arthroscopic surgery and latest follow-up.

Radiographic Outcomes and Arthroscopic Findings

The mean pre- and postoperative hip-knee-ankle angle, PTS, MPTA, and JLCA of the knee joints are summarized in Table 3. Postoperatively, the MPTA and PTS values were significantly higher in the MD-ACL group as compared with the control group (P = .034 and P = .021, respectively). In the multivariable regression analysis of potential predictive factors, a smaller notch width index (P = .003), an increased postoperative MPTA (P = .031), and a larger ΔPTS (P = .004) were identified as independent predictors of MD-ACL development after HTO (Table 4).

Table 3

Radiologic Outcomes Between the MD-ACL and Control Groups ^a

	Mean ± SD or No. (%)
	MD-ACL (n = 43)	Control (n = 43)	P Value ^b
HKA angle, deg ^c
Preoperative	6.8 ± 2.4	7.2 ± 2.8	.462
Postoperative	−2.7 ± 1.2	−2.8 ± 1.8	.867
MPTA, deg
Preoperative	85.2 ± 1.7	85.3 ± 2.0	.901
Postoperative	94.2 ± 3.0	93.1 ± 2.3	.034
JLCA, deg
Preoperative	4.2 ± 1.5	4.1 ± 1.8	.838
Postoperative	2.1 ± 1.2	2.0 ± 1.3	.958
PTS, deg
Preoperative	8.1 ± 2.1	8.2 ± 2.0	.970
Postoperative	11.3 ± 3.0	10.1 ± 2.1	.021
Preoperative Kellgren-Lawrence grade			.550
2/3/4	13/29/1	12/28/3
Initial cartilage status ^d			.387
Grade 1	2 (4.6)	5 (11.6)
Grade 2	7 (16.2)	3 (7.0)
Grade 3	17 (39.6)	19 (44.2)
Grade 4	17 (39.6)	16 (37.2)

HKA, hip-knee-ankle; JLCA, joint line convergence angle; MD-ACL, mucoid degeneration of the anterior cruciate ligament; MPTA, medial proximal tibial angle; PTS, posterior tibial slope.

An independent t test was used to analyze differences in radiographic results, while a chi-square test was used to analyze differences in initial cartilage status. Bold indicates a statistically significant difference (P < .05).

A positive angle represents varus alignment, and a negative angle represents valgus alignment.

Initial arthroscopic cartilage status was graded according to the International Cartilage Repair Society grading system.

Table 4

Univariable and Multivariable Logistic Regression Analysis for Progressive MD-ACL After HTO ^a

	Univariate Analysis		Multivariate Analysis
Variable	Estimate (95% CI)	P Value	Estimate (95% CI)	P Value
Age	−0.020 (−1.143 to 0.097)	.952
Sex: female vs male	−0.181 (−2.502 to 1.232)	.764
Body mass index	−0.020 (−0.187 to 0.262)	.714
Interval between HTO and second-look arthroscopy	−0.002 (−0.070 to 0.061)	.895
Preoperative
Kellgren-Lawrence grade	−0.252 (−2.316 to 0.688)	.540
HKA angle	−0.061 (−0.530 to 0.182)	.457
MPTA	−0.016 (−0.744 to 0.206)	.899
JLCA	0.027 (−0.284 to 0.740)	.836
PTS	−0.003 (−1.385 to −0.253)	.978
Notch width index	−0.454 (−0.820 to −0.159)	.001	−0.476 (−0.817 to −0.184)	.003
Initial cartilage status	0.113 (−0.913 to 0.308)	.596
Postoperative
HKA angle	0.024 (−0.115 to 0.904)	.865
MPTA	0.198 (0.119 to 0.766)	.038	0.243 (0.031 to 0.477)	.031
JLCA	−0.009 (−0.816 to 0.441)	.958
PTS	0.215 (0.246 to 1.160)	.025	0.043 (−0.315 to 0.230)	.750
ΔPTS	0.612 (0.246 to 1.160)	.001	0.679 (−0.234 to 1.179)	.004

Bold indicates P < .05 (statistically significant difference). HKA, hip-knee-ankle; HTO, high tibial osteotomy; JLCA, joint line convergence angle; MD-ACL, mucoid degeneration of the anterior cruciate ligament; MPTA, medial proximal tibial angle; PTS, posterior tibial slope.

Survival

The overall probability of survival was 97.6% (95.3% in the MD-ACL group and 100% in the control group; Figure 6) over a mean follow-up of 72.5 months. Conversion to total knee arthroplasty was required in 2 patients (4.7%) in the MD-ACL group owing to persistent symptoms, whereas no conversions were observed in the control group. There were no statistically significant differences in survival rates between the groups (P = .400, log-rank test).

Figure 6.

Kaplan-Meier survival curves (with 95% CI) for high tibial osteotomy with total knee arthroplasty as the endpoint. ACL, anterior cruciate ligament; MD, mucoid degeneration.

Discussion

The principal findings of this study were that the postoperative development of MD-ACL occurred in 13.9% of cases and was significantly associated with adverse postoperative outcomes after HTO over a mean follow-up of 72.5 months. Patients with postoperative MD-ACL exhibited significantly worse clinical outcomes as compared with the control group, with further deterioration observed from the time of second-look arthroscopic surgery to the latest follow-up. During this period, a significant decline was observed only in IKDC score, whereas other clinical parameters remained stable without notable changes. In contrast, clinical outcomes in the control group remained stable, demonstrating sustained improvement over the same period. Moreover, the study identified smaller intercondylar notch geometry, increased postoperative MPTA, and larger ΔPTS as predisposing factors for MD-ACL after HTO. Our findings highlight the importance of careful preoperative planning and meticulous attention to surgical techniques that avoid excessive increases in MPTA and PTS, thereby improving HTO outcomes. Orthopaedic surgeons should thoroughly evaluate these factors to optimize clinical outcomes in patients undergoing HTO for medial compartmental osteoarthritis.

The benefits of HTO in achieving significant improvements in clinical and patient-reported outcomes, with satisfactory survival rates, have been well documented.^{8,15,33,44,53} However, the potential relationship between degenerative changes of the ACL and inferior outcomes has been a growing concern in the literature.^28,39,40 MD-ACL is increasingly recognized as a pathologic condition, as characterized by degeneration of collagen fibers, infiltration of mucoid-like material, and associated biomechanical consequences (eg, notch impingement, restricted range of motion, posterior knee pain). Biomechanical research by Giffin et al¹⁸ demonstrated that increased tibial slope leads to an anterior shift in the tibial resting position, suggesting that an increased postoperative PTS after HTO amplifies strain on the ACL through anterior tibial translation.

The present data demonstrated a significant difference in KOOS subscales and the IKDC score between the groups at an intermediate follow-up. Furthermore, a progressive decline in clinical scores from the time of second-look arthroscopic surgery to the latest follow-up was observed exclusively in the MD-ACL group, whereas the control group exhibited stable improvements in clinical scores over the same period. These results indicate that the development of MD-ACL may be a critical determinant of postoperative HTO outcomes. In contrast, a short-term retrospective study by Kim et al²⁸ reported no correlation between ACL degeneration observed during second-look arthroscopy and clinical outcomes after HTO. However, the findings from our study underscore the necessity for a follow-up period >2 years to fully elucidate the effect of ACL degeneration on postoperative HTO outcomes. Considering that ACL degeneration significantly increases the risk of subsequent meniscal tears and osteoarthritis, minimizing MD-ACL should be prioritized to optimize long-term outcomes.^20,22,32,34 Despite the association of MD-ACL with inferior clinical outcomes after HTO, we achieved an overall survival rate of 97.6% over a mean follow-up of 72.5 months, consistent with previously reported results.^6,8,15,16,54 These findings highlight the durability of clinical improvements achieved through HTO, even in the presence of MD-ACL at the time of second-look arthroscopy.

The pathogenesis of MD-ACL remains incompletely understood; however, several hypotheses have been proposed, including infiltration of synovial fluid within ACL fibers, age-related degenerative changes, a history of trauma, and the presence of ectopic synovial tissue within the ACL structure. While a definitive consensus has not been established, altered biomechanics associated with specific bony morphologies of the knee joint have been suggested as potential contributors to MD-ACL. Specifically, a steeper PTS may theoretically increase anterior tibial translation, thereby elevating tensile forces on the ACL.³ This biomechanical alteration may adversely affect ACL function and knee kinematics.^14,42 Additionally, several studies have identified a correlation between intercondylar notch geometry and the presence of MD-ACL.^{4,11,21,29,34} For instance, Cha et al¹¹ demonstrated that a steeper notch angle and a narrower intercondylar notch are significantly associated with MD-ACL. This anatomic configuration, characterized by a smaller and narrower intercondylar notch, has been proposed as a contributing factor to MD-ACL, potentially attributed to ligament impingement against the notch roof and lateral walls,^24,38 as well as increased collagen remodeling and sulfated glycosaminoglycan deposition.¹² Furthermore, Ogawa et al³⁹ identified a relationship between changes in MPTA and ACL deterioration after HTO, suggesting that increases in MPTA may adversely affect ACL integrity by reducing tibial subluxation and thereby promoting medial translation of the tibial attachment. Although MPTA serves as a reliable indicator of the degree of correction achieved during HTO, a direct causal relationship between MPTA and joint line obliquity has not been conclusively demonstrated. In the present study, no significant differences were observed in pre- or postoperative JLCA between the MD-ACL and control groups, suggesting that JLCA was not a contributing factor in the development of MD-ACL after HTO. Song et al⁵⁰ reported that a postoperative joint line obliquity ≥4° was associated with poorer clinical outcomes, while ≥6° negatively affected radiologic outcomes after HTO at a mean follow-up of 55.0 months. Taken together, these findings imply that increased MPTA may contribute to inferior outcomes by modifying joint line obliquity, although further research is needed to establish a definitive causal relationship. The clinical implications of these findings underscore the importance of exploring a potential causal relationship between HTO and the development of MD-ACL, as well as its effect on clinical outcomes. Our findings demonstrated significant associations between the development of MD-ACL and poorer clinical outcomes after HTO. While smaller intercondylar notch geometry increased MPTA and a larger ΔPTS during HTO may influence knee biomechanics and contribute to ACL degeneration and clinical outcomes, these factors may represent confounding variables rather than direct causal pathways. Prospective studies are warranted to elucidate the interplay among altered knee biomechanics, ACL strain, and anatomic predispositions, as well as their effect on clinical outcomes.

Limitations

This study has several limitations. First, as a retrospective analysis, patient selection was not randomized. Second, the study included only patients who consented to plate removal and second-look arthroscopy, further contributing to selection bias. In practice, second-look arthroscopy was routinely recommended to patients at the time of plate removal to assess intra-articular status; however, only those who consented to these follow-up procedures were included in this analysis. Consequently, we recognize the potential for selection bias in the allocation of groups within the current study. Third, although this study primarily relied on consistent findings between MRI and arthroscopy for diagnosing MD-ACL, statistical quantification of agreement between these modalities was not performed. Furthermore, despite demonstrating good inter- and intraobserver agreement, the potential for subjectivity in arthroscopic assessment remains. Future investigations are warranted to analyze the reliability and validity of MRI as compared with arthroscopic findings for the detection of MD-ACL based on established statistical methods. Fourth, although a matched-controlled design was employed to minimize selection bias, it remains possible that additional variables, such as preoperative Kellgren-Lawrence grade and patient-reported outcome measures, could have been incorporated into the matching process and may have influenced the outcomes. However, the inclusion of additional matching variables may have substantially reduced the matched sample size, particularly given the limited number of MD-ACL cases in the present study. Further prospective investigations involving larger matched cohorts and a broader set of unmatched variables are warranted to validate these associations and clarify their clinical relevance. Nevertheless, propensity score matching remains a robust method for reducing selection bias in the context of retrospective study designs, and in this study, matching was performed with common preoperative parameters, including age, sex, body mass index, and the interval between HTO and second-look arthroscopy, to enhance cohort comparability. Fifth, the exclusion of patients >65 years old may limit the generalizability of our findings to older populations and may also introduce selection bias. Finally, although the predisposing factors included in the multivariate logistic regression analysis were selected by clinical relevance and previous literature,^{3,11,21,28,39,42} the influence of unmeasured confounding factors on MD-ACL development cannot be ruled out. Despite these limitations, the study provides valuable insights into the intermediate-term outcomes and survival rates after HTO, supported by a relatively large case series with a control group.

Conclusion

The development of MD-ACL is associated with adverse postoperative outcomes after HTO, with a mean follow-up of 72.5 months. Specifically, the results indicate that smaller intercondylar notch geometry, increased MPTA, and a larger ΔPTS during HTO are significant predictors for the development of MD-ACL. Our findings highlight the importance of careful preoperative planning and meticulous attention to surgical techniques that avoid excessive increases in MPTA and PTS, thereby improving HTO outcomes. Orthopaedic surgeons should thoroughly evaluate these factors to optimize clinical outcomes in patients undergoing HTO for medial compartmental osteoarthritis.

Footnotes

Final revision submitted July 25, 2025; accepted September 8, 2025.

The authors have declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from Chonnam National University Hwasun Hospital (IRB No. CNUHH-2024-197).

ORCID iDs

Hong Yeol Yang

Jong Keun Seon

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Development of Mucoid Degeneration of the Anterior Cruciate Ligament Is Associated With Intermediate-Term Outcomes After High Tibial Osteotomy: A Propensity Score–Matched Analysis

Abstract

Background:

Purpose:

Study Design:

Methods:

Results:

Conclusion:

Keywords

Methods

Patients

Surgical Technique and Postoperative Rehabilitation

Clinical Evaluation

Radiographic Evaluation

MRI Assessment and Arthroscopic Evaluation

Predictive Factors Associated With the Development of MD-ACL

Statistical Analysis

Results

Clinical Outcomes

Radiographic Outcomes and Arthroscopic Findings

Survival

Discussion

Limitations

Conclusion

Footnotes

ORCID iDs

References