Relationship Between Quadriceps Strength at 6 Months Postoperatively and Improvement in Patient-Reported Knee Function After Anterior Cruciate Ligament Reconstruction

Abstract

Background:

Understanding the factors associated with poor recovery over time after anterior cruciate ligament reconstruction (ACLR) helps clinicians identify patients who are at risk and targets for an intervention.

Purpose:

To determine the factors associated with improvement in subjective knee function from 6 to 12 months after ACLR.

Study Design:

Case-control study; Level of evidence, 3.

Methods:

A total of 91 patients undergoing primary unilateral ACLR were included. Subjective knee function was assessed using the International Knee Documentation Committee Subjective Knee Form (IKDC-SKF) at 6 and 12 months postoperatively. Isokinetic knee strength (quadriceps and hamstring) and Anterior Cruciate Ligament–Return to Sport After Injury (ACL-RSI) scale scores were also assessed. Patients were included in the poor recovery group if their improvement in the IKDC-SKF score from 6 to 12 months was <15.5 points (minimal detectable change) and their IKDC-SKF score at 12 months was <90 points (maximum, 100 points). The IKDC-SKF score, knee strength, and the ACL-RSI score were compared between the poor recovery and good recovery groups with and without propensity score matching. Matched variables included age, sex, and IKDC-SKF score at 6 months after ACLR. In addition, logistic regression analysis was performed to identify factors that discriminated between the poor recovery and good recovery groups.

Results:

There were 32 participants (35%) allocated to the poor recovery group. Before propensity score matching, the poor recovery group had a significantly older age, lower IKDC-SKF scores at 6 and 12 months, and a lower limb symmetry index (LSI) for quadriceps strength at 6 months. After propensity score matching, the LSI for quadriceps strength at 6 months was significantly different between the poor recovery and good recovery groups (73.0 ± 17.4 vs 83.3 ± 18.2, respectively; P = .039). Logistic regression analysis showed that a lower LSI for quadriceps strength at 6 months was significantly associated with poor recovery of the IKDC-SKF score (odds ratio, 0.96 [95% CI, 0.93-0.98]), and receiver operating characteristic curve analysis identified 80.9% as a cutoff value of the LSI for quadriceps strength with 75.0% sensitivity and 61.0% specificity.

Conclusion:

A lower LSI for quadriceps strength at 6 months postoperatively was associated with poor recovery of the IKDC-SKF score from 6 to 12 months after ACLR, even after adjusting for confounders.

Keywords

knee ligaments ACL clinical assessments/grading scales patient-reported outcome measures psychological readiness rehabilitation subgroup analysis

Patient-reported outcomes are one of the most important outcomes after anterior cruciate ligament (ACL) reconstruction (ACLR). The International Knee Documentation Committee (IKDC) Subjective Knee Form (IKDC-SKF) is the most commonly used patient-reported outcome measure and assesses knee symptoms, knee function, and sports activities, with higher scores indicating better subjective knee function.¹¹ Patients with greater satisfaction after ACLR have higher IKDC-SKF scores than those with lower satisfaction.¹⁷ In addition, the IKDC-SKF score after ACLR has been used as one of the criteria for return to sports (RTS).^35,36 Patients who are able to return to their preinjury level of sports after ACLR have a higher IKDC-SKF score than those who are unable to return to their preinjury level of sports.²¹

The IKDC-SKF score after ACLR improves over the postoperative period.³ However, improvement in subjective knee function over time has been reported to be variable. Approximately half of patients have an IKDC-SKF score <90 (maximum score, 100) at 12 months postoperatively, which does not meet the criteria for RTS.^35,36 Patients who returned to their preinjury level of sports showed clinically relevant improvement in the IKDC-SKF score from 10 to 25 months after ACLR, whereas patients who did not return to their preinjury level of sports showed no meaningful change in the IKDC-SKF score over the same period.³⁹ A recent study showed 4 trajectories of the IKDC-SKF score after ACLR, and 8% and 51.4% of patients were assigned to low and moderate recovery trajectories, respectively.²⁷ Understanding the factors associated with poor recovery over time after ACLR could help clinicians identify patients who are at risk and targets for an intervention. However, the characteristics of patients with poor recovery in subjective knee function over time after ACLR are not well understood.

Previous cross-sectional studies have shown that better IKDC-SKF scores after ACLR are associated with greater quadriceps strength and greater psychological readiness to RTS.^4,6,20,29,42 However, it was not clear whether these factors were also associated with improvement in the IKDC-SKF score over time after ACLR. The early recovery of quadriceps strength is associated with better psychological readiness to RTS and return to preinjury levels of performance.^33,34 Characterizing patients with less improvement in the IKDC-SKF score after ACLR will contribute to the development of rehabilitation to achieve good subjective knee function after ACLR.

The purpose of this study was to determine the factors associated with improvement in the IKDC-SKF score from 6 to 12 months after ACLR. The hypothesis was that knee muscle strength and psychological readiness to RTS would be associated with improvement in the IKDC-SKF score from 6 to 12 months after ACLR.

Methods

Participants

After approval from our institutional review board, this case-control study retrospectively collected data from the medical records of patients who underwent primary unilateral ACLR at an orthopaedic hospital between November 2017 and December 2022 (Figure 1). Patients were excluded based on the following criteria: time interval between the ACL injury and ACLR >1 year, no participation in IKDC sports level I or II before the ACL injury, no desire to return to level I or II sports, history of lower extremity surgery other than ACLR, or cartilage injuries requiring additional surgery. Patients who underwent concomitant meniscal surgery, either partial meniscectomy or meniscal repair, were included. A total of 194 patients were eligible, and 91 patients who were evaluated at 6 and 12 months were included in the present study (45 male and 46 female; mean age, 23.4 ± 10.9 years).

Figure 1.

Flowchart of study participants. ACL, anterior cruciate ligament; ACLR, ACL reconstruction; IKDC, International Knee Documentation Committee.

Surgical Technique and Rehabilitation

All patients underwent anatomic double-bundle ACLR with a hamstring tendon autograft and completed a standardized rehabilitation protocol. ACLR procedures were performed by 3 experienced surgeons (S.K., C.I. and Y.A.) according to previously reported procedures.^40,41 Harvested semitendinosus and gracilis tendon autografts connected with polyester tape (Leeds-Keio artificial ligament; Neoligaments) and EndoButton CL BTB (Smith+Nephew) were used as ligament substitutes. Next, 2 tibial bone tunnels for the anteromedial and posterolateral bundles were created using a cannulated drill. Then, 2 femoral sockets were created at the center of the femoral attachment of the anteromedial and posterolateral bundles using the transtibial technique. For meniscal injuries, meniscal repair or partial meniscectomy was performed depending on the location, size, and type of tear.

Range of motion, quadriceps setting, and straight-leg raising exercises were started after drainage tube removal.^13,34 Knee flexion was limited to 90° and 120° until 4 and 6 weeks postoperatively, respectively. Full weightbearing was allowed as tolerated at 1 week postoperatively. Closed kinetic chain exercises were started at 2 weeks postoperatively with double-leg squatting and progressed to split-stance squatting and single-leg squatting according to the patient’s symptoms such as swelling and pain, muscle strength, and squatting technique. Hip and trunk muscle strengthening was also performed. In patients who underwent meniscal repair, the initiation of weightbearing and range of motion exercises was delayed by 1 or 2 weeks depending on the location, size, and type of tear. Running and sport-specific exercises were started gradually at 12 weeks and 5 months, respectively. Full RTS was generally allowed at 9 months after surgery. Rehabilitation was supervised by physical therapists, and progression was adjusted according to the patient’s symptoms and function.

IKDC-SKF

Patient-reported knee function was assessed using the IKDC-SKF at 6 and 12 months after ACLR. This outcome measure consists of 18 items for knee symptoms, function, and sports activity, with scores ranging from 0 (worst) to 100 (best).¹¹ The IKDC-SKF is highly relevant and reliable,¹¹ and the Japanese version of the IKDC-SKF used in this study has been reported to have high validity and reliability.¹⁰

Knee Strength Testing

Isokinetic quadriceps and hamstring strength were assessed using a dynamometer (Biodex System 3; Biodex Medical Systems) at 6 and 12 months after ACLR.^13,33 Participants were secured to the dynamometer with their hips flexed to 90°. Concentric knee extension and flexion torques were measured at 60 deg/s. Knee strength testing was performed with 5 repetitions at maximal effort after a few submaximal practice trials. Peak knee extension and flexion torques were recorded, and the limb symmetry index (LSI) was calculated as the percentage of the involved limb to the uninvolved limb.

Psychological Readiness to RTS

The Anterior Cruciate Ligament–Return to Sport After Injury (ACL-RSI) scale was used to assess psychological readiness to RTS at 6 and 12 months postoperatively. The ACL-RSI scale consists of 12 items and includes 3 domains: emotions, confidence in performance, and risk appraisal.³⁷ The ACL-RSI score ranges from 0 to 100, with higher scores indicating a better state of psychological readiness to RTS. The Japanese version of the ACL-RSI scale used in this study has shown high validity and reliability.⁹

RTS Level

RTS level was assessed at 12 months postoperatively. Patients selected their current RTS status using the following Likert scale: (1) “≥pre”: return to the same sports as before the injury, at or above the preinjury level; (2) “<pre”: return to the same sports as before the injury, but not to the desired level; (3) “other sports”: return to some sports activity, but not to the same sports as before the injury (or desired sports); or (4) “no RTS.”^2,33 In addition, patients were asked about their reasons for not returning to preinjury levels: (1) fear of reinjuries, (2) functional limitations of the knee, (3) social reasons (progressing in school, moving, busy at work), and/or (4) other.³³

Statistical Analysis

Data are presented as the mean and standard deviation. First, we confirmed the associations between improvements in outcomes (IKDC-SKF score, quadriceps and hamstring strength, and ACL-RSI score) from 6 to 12 months after ACLR using Pearson correlation analysis. Second, we conducted subgroup analysis, as a good IKDC-SKF score at 6 months may result in a smaller change from 6 to 12 months because of the ceiling effect. We defined the poor recovery group according to the following 2 criteria:

Improvement in the IKDC-SKF score from 6 to 12 months after ACLR is <15.5 points because the minimal detectable change has been reported as 13.7 to 15.5.^7,10

The IKDC-SKF score at 12 months after ACLR is <90 points, which is used as one of the criteria for RTS^35,36 and good recovery.^29,42

Matched controls (good recovery group) were then selected using propensity score matching at a 1:1 ratio according to nearest neighbor matching without replacement within a caliper width of 0.2. To estimate the propensity score, logistic regression analysis was used for covariates including age, sex, and IKDC-SKF score at 6 months after ACLR because these variables have been reported as potential confounders for improvement in the IKDC-SKF score after ACLR.²⁷ The balance of covariates between groups was assessed by calculating the standardized mean difference.²³

A priori sample size calculation showed that a total of 34 patients was needed to achieve a significance level (α), statistical power (1 −β), and an effect size (Cohen d) of 0.05, 0.80, and 1.0, respectively. Before and after propensity score matching, an independent t test and the chi-square test were used to examine continuous and categorical variables between the poor recovery group and the good recovery group, respectively. Furthermore, the Cohen d value was calculated as the effect size.⁵ A Cohen d value ≥0.80 was interpreted as large, 0.50-0.79 as moderate, and 0.20-0.49 as small.⁵ Univariate logistic regression analysis was used to examine independent variables to differentiate between the poor recovery and good recovery groups. The independent variables were LSI for quadriceps and hamstring strength at 6 months and ACL-RSI score at 6 months. When significant independent variables were found, robustness was assessed by adjusting for age, sex, and IKDC-SKF score at 6 months. Receiver operating characteristic (ROC) curve analysis was used to determine the area under the curve (AUC) and cutoff value for the independent variables. Sensitivity and specificity were calculated. The AUC was classified according to the following criteria based on a previous study: fail (0.50-0.59), poor (0.60-0.69), fair (0.70-0.79), good (0.80-0.89), and excellent (0.90-1.00).¹⁹ Statistical significance was set at P < .05. Statistical analyses were performed using JMP Pro 16 (SAS Institute).

Results

Pearson correlation analysis revealed a significant positive relationship between improvement in the IKDC-SKF score and ACL-RSI score from 6 to 12 months postoperatively (R = 0.315; P = .002). Improvement in the LSI for quadriceps strength was also correlated with improvement in the LSI for hamstring strength (R = 0.362; P < .001). There was no other significant correlation or relationship between improvements in outcomes (|R| = 0.045-0.099; P≥ .352).

Of the 91 participants, 32 participants (14 male, 18 female) met the criteria for the poor recovery group. Before propensity score matching, significant group differences were found for age, concomitant meniscectomy, IKDC-SKF score at 6 and 12 months, LSI for quadriceps strength at 6 months, and ACL-RSI score at 12 months (Table 1). The proportion of RTS “≥pre” was higher in the good recovery group than in the poor recovery group, but the difference was not statistically significant (P = .073). Propensity score matching selected 27 patients each for the poor recovery group and matched good recovery group (Table 2). The IKDC-SKF score at 12 months was significantly lower in the poor recovery group than in the matched good recovery group (P < .001; d = 1.600), whereas the IKDC-SKF score at 6 months showed no significant group difference (P = .998). Even after propensity score matching, the poor recovery group had a significantly lower LSI for quadriceps strength at 6 months than the matched good recovery group (P = .039; d = 0.580). The proportion of patients who underwent concomitant meniscal surgery was significantly greater in the poor recovery group than in the matched good recovery group (P = .028; d = 0.330). There were no other significant group differences after propensity score matching (Table 2).

Table 1

Patient Characteristics Without Propensity Score Matching^a

	Poor Recovery Group (n = 32)	Good Recovery Group (n = 59)	d or φ	P
Age, y	27.3 ± 12.4	21.3 ± 9.6	0.470	.013
Female sex	18 (56)	28 (47)	0.080	.512
Body mass index, kg/m²	23.7 ± 3.0	22.7 ± 4.4	0.250	.264
IKDC sports level	I: 21 (66); II: 11 (34)	I: 48 (81); II: 11 (19)	0.180	.125
Collegiate	4 (13)	7 (12)	0.010	>.999
High school	8 (25)	29 (49)	0.240	.028
Junior high school	3 (9)	8 (14)	0.060	.741
Recreational	17 (53)	15 (25)	0.280	.012
Concomitant meniscal surgery	20 (63)	25 (42)	0.190	.081
Meniscal repair	11 (34)	22 (37)	0.030	.823
Meniscectomy	9 (28)	3 (5)	0.330	.003
Complications (wound infection)	0 (0)	1 (2)	0.080	>.999
IKDC-SKF score
Preoperative^b	70.1 ± 12.3	68.4 ± 13.3	0.570	.014
≥90 points	0 (0)	1 (2)
6 mo	76.1 ± 8.3	81.3 ± 12.9	0.450	.045
≥90 points	2 (6)	17 (29)
12 mo	80.5 ± 8.4	94.6 ± 6.2	2.000	<.001
≥90 points	0 (0)	52 (88)
Change from 6 to 12 mo	4.4 ± 9.2	13.4 ± 9.4	0.960	<.001
≥15.5 points	0 (0)	24 (41)
LSI for quadriceps strength, %
6 mo	72.1 ± 16.6	82.9 ± 15.0	0.690	.002
12 mo	87.0 ± 13.0	91.5 ± 12.3	0.360	.105
LSI for hamstring strength, %
6 mo	84.2 ± 23.1	84.4 ± 14.1	0.010	.966
12 mo	94.8 ± 13.9	91.7 ± 12.7	0.230	.289
ACL-RSI score
6 mo	56.0 ± 20.7	61.3 ± 22.2	0.240	.272
12 mo	60.4 ± 20.5	73.3 ± 23.6	0.570	.011
RTS at 12 mo^c			0.310	.073
≥Pre	5 (16)	19 (32)
<Pre	15 (47)	23 (39)
Other sports	1 (3)	0 (0)
No RTS (functional limitation)	3 (9)	1 (2)
No RTS (social reason)	7 (22)	7 (12)

Data are shown as mean ± SD or n (%). Boldface indicates statistical significance (P < .05). ACL-RSI, Anterior Cruciate Ligament–Return to Sport After Injury; IKDC, International Knee Documentation Committee; IKDC-SKF, International Knee Documentation Committee Subjective Knee Form; LSI, limb symmetry index; ≥Pre, return to the same sports as before the injury, at or above the preinjury level; RTS, return to sports.

Data were missing for 1 patient in the poor recovery group (3%) and 7 patients in the good recovery group (12%).

One patient in the poor recovery group (3%) and 9 patients in the good recovery group (15%) returned to the same sports as before the injury, but whether they were “≥pre” or “<pre” is unknown.

Table 2

Patient Characteristics With Propensity Score Matching^a

	Poor Recovery Group (n = 27)	Matched Good Recovery Group (n = 27)	d or φ	P	SMD^b
Age, y	24.7 ± 11.4	23.6 ± 10.9	0.100	.716	0.009
Female sex	14 (52)	16 (59)	0.080	.785	0.149
Body mass index, kg/m²	23.2 ± 2.3	22.6 ± 5.0	0.150	.584
IKDC sports level	I: 19 (70); II: 8 (30)	I: 21 (78); II: 6 (22)	0.090	.757
Collegiate	4 (15)	2 (7)	0.120	.669
High school	8 (30)	10 (37)	0.080	.773
Junior high school	3 (11)	4 (15)	0.060	>.999
Recreational	12 (44)	11 (41)	0.040	>.999
Concomitant meniscal surgery	17 (63)	8 (30)	0.330	.028
Meniscal repair	11 (41)	7 (26)	0.160	.387
Meniscectomy	6 (22)	1 (4)	0.280	.100
Complications	0 (0)	0 (0)	—	—
IKDC-SKF score
Preoperative^c	61.4 ± 12.0	67.3 ± 12.2	0.490	.092
≥90 points	0 (0)	0 (0)
6 mo	76.9 ± 8.2	76.9 ± 13.9	0.000	.998	<0.001
≥90 points	2 (7)	4 (15)
12 mo	80.4 ± 8.4	93.4 ± 7.8	1.600	<.001
≥90 points	0 (0)	21 (78)
Change from 6 to 12 mo	3.6 ± 9.7	16.5 ± 9.2	1.370	<.001
≥15.5 points	0 (0)	14 (52)
LSI for quadriceps strength, %
6 mo	73.0 ± 17.4	83.3 ± 18.2	0.580	.039
12 mo	87.4 ± 13.9	88.5 ± 15.4	0.040	.872
LSI for hamstring strength, %
6 mo	82.7 ± 13.4	83.2 ± 17.2	0.030	.912
12 mo	96.8 ± 13.4	92.2 ± 10.6	0.380	.168
ACL-RSI score
6 mo	57.3 ± 19.5	61.2 ± 21.1	0.190	.483
12 mo	60.6 ± 19.7	70.7 ± 24.6	0.450	.102
RTS at 12 mo^d			0.350	.184
≥Pre	5 (19)	8 (30)
<Pre	13 (48)	12 (44)
Other sports	0 (0)	0 (0)
No RTS (functional limitation)	2 (7)	0 (0)
No RTS (social reason)	7 (26)	4 (15)

Data are shown as mean ± SD or n (%). Boldface indicates statistical significance (P < .05). Dashes indicate statistical test not applicable. ACL-RSI, Anterior Cruciate Ligament–Return to Sport After Injury; IKDC, International Knee Documentation Committee; IKDC-SKF, International Knee Documentation Committee Subjective Knee Form; LSI, limb symmetry index; ≥Pre, return to the same sports as before the injury, at or above the preinjury level; RTS, return to sports; SMD, standardized mean difference.

SMD <0.1 indicates an adequate variable balance after propensity score matching.

Data were missing for 1 patient in the poor recovery group (4%) and 3 patients in the good recovery group (11%).

Three patients in the good recovery group (11%) returned to the same sports as before the injury, but whether they were “≥pre” or “<pre” is unknown.

Univariate logistic regression analysis showed that a lower LSI for quadriceps strength at 6 months and concomitant meniscectomy were associated with the poor recovery group (Table 3). Even after adjusting for age, sex, and IKDC-SKF score at 6 months, the LSI for quadriceps strength at 6 months was significantly associated with poor recovery (odds ratio, 0.958 [95% CI, 0.927-0.991]; P = .013). ROC curve analysis revealed that the AUC of the LSI for quadriceps strength at 6 months was 0.693. The cutoff value to distinguish the poor recovery group from the good recovery group was 80.9% (sensitivity: 75.0%; specificity: 61.0%) (Figure 2). Concomitant meniscectomy was also associated with poor recovery, even after adjusting for age, sex, and IKDC-SKF score at 6 months (odds ratio, 5.697 [95% CI, 1.325-24.503]; P = .019).

Table 3

Independent Factors Associated With Poor Recovery of IKDC-SKF Score From 6 to 12 Months (n = 91)^a

	B (95% CI)	OR (95% CI)	P
Age, y	0.05 (0.01 to 0.09)	1.05 (1.01 to 1.10)	.018
Sex (0: male; 1: female)	−0.18 (−0.62 to 0.25)	1.42 (0.60 to 3.42)	.424
Body mass index, kg/m²	0.06 (−0.05 to 0.18)	1.06 (0.95 to 1.20)	.271
IKDC sports level (0: I; 1: II)	−0.41 (−0.91 to 0.08)	2.29 (0.85 to 6.17)	.098
Concomitant meniscal surgery	−0.41 (−0.86 to 0.03)	2.27 (0.95 to 5.60)	.069
Meniscal repair	0.06 (−0.38 to 0.52)	0.88 (0.35 to 2.15)	.783
Meniscectomy	−0.99 (−1.78 to −0.34)	7.30 (1.98 to 35.20)	.005
IKDC-SKF score at 6 mo	−0.04 (−0.08 to 0.00)	0.96 (0.92 to 1.00)	.050
LSI for quadriceps strength at 6 mo	−0.05 (−0.08 to −0.02)	0.96 (0.93 to 0.98)	.004
LSI for hamstring strength at 6 mo	0.00 (−0.03 to 0.02)	1.00 (0.97 to 1.02)	.965
ACL-RSI score at 6 mo	−0.01 (−0.03 to 0.01)	0.99 (0.97 to 1.01)	.270

Boldface indicates statistical significance (P < .05). ACL-RSI, Anterior Cruciate Ligament–Return to Sport After Injury; B, partial regression coefficient; IKDC, International Knee Documentation Committee; IKDC-SKF, International Knee Documentation Committee Subjective Knee Form; LSI, limb symmetry index; OR, odds ratio.

Figure 2.

Receiver operating characteristic (ROC) curve analysis to discriminate poor improvement in the International Knee Documentation Committee subjective knee form score from 6 to 12 months after anterior cruciate ligament reconstruction.

Discussion

This study investigated the factors associated with improvement in the IKDC-SKF score from 6 to 12 months after ACLR. The main finding of the present study was that a lower LSI for quadriceps strength at 6 months was associated with poor improvement in the IKDC-SKF score from 6 to 12 months after ACLR, even after adjusting for confounders including age, sex, and IKDC-SKF score at 6 months. The findings indicated that quadriceps strength at 6 months was associated with improvement in subjective knee function from 6 to 12 months after ACLR.

Both the poor recovery group and the matched good recovery group showed comparable or better IKDC-SKF scores at 6 and 12 months compared to the range of 74.2 to 75.9 points previously reported as the patient acceptable symptom state.^24,28 However, the poor recovery group did not show clinically meaningful improvement from 6 to 12 months and did not reach a score of 90 points at 12 months, which is often used as criteria for RTS^35,36 and good recovery.^29,42 The present results suggest that the poor recovery group may benefit from an additional intervention before and/or after 6 months to improve knee function.

The 2 matched groups showed a significant difference in the LSI for isokinetic quadriceps strength at 6 months, and logistic regression analysis showed that a lower LSI for quadriceps strength at 6 months was significantly associated with low improvement in the IKDC-SKF score from 6 to 12 months after ACLR, even after adjusting for confounders. These results partially supported our hypothesis that knee muscle strength would be associated with improvement in the IKDC-SKF score from 6 to 12 months after ACLR. Previous cross-sectional studies have reported that the LSI for quadriceps strength is positively correlated with the IKDC-SKF score after ACLR.^4,6,20,29,42 The present study showed that the low LSI for quadriceps strength at 6 months was also associated with no clinically meaningful improvement in the IKDC-SKF score from 6 to 12 months after ACLR. The assessment of quadriceps strength at 6 months postoperatively would be more important in prognosticating subsequent recovery of subjective knee function than the IKDC-SKF score itself. The LSI for quadriceps strength at 3 months has also been demonstrated to be significantly associated with the Lysholm score at 6 months after ACLR.³⁰ A recent study showed that the LSI for isokinetic quadriceps strength at 6 months was associated with RTS at preinjury levels at 12 months postoperatively.³³ Many studies have reported that quadriceps strength affects asymmetrical motion patterns with less load on the reconstructed knee during sports activities.^{12,13,15,16,26,31} To achieve an IKDC-SKF score ≥90, patients must be able to perform very strenuous activities such as jumping or pivoting as in basketball or soccer.¹¹ In this study, the proportion of RTS “≥pre” was higher in the good recovery group than in the poor recovery group, although no statistically significant difference was found. Therefore, quadriceps strength at 6 months postoperatively may be associated with subsequent improvement in the IKDC-SKF score.

ROC curve analysis showed that the cutoff value of the LSI for quadriceps strength at 6 months was 80.9% to discriminate poor improvement in the IKDC-SKF score from 6 to 12 months after ACLR. A previous study showed that patients with a ≥90% LSI for quadriceps strength at the time of RTS (7-8 months postoperatively) had a better IKDC-SKF score at 12 months than those with a <85% LSI for quadriceps strength,¹⁴ which is supported by the present results after adjusting for confounders. In addition, the LSI for quadriceps strength at 6 months postoperatively has been reported as 92.9 ± 9.1 and 76.5 ± 16.0 in patients who returned to sports at preinjury levels and below preinjury levels at 12 months after ACLR, respectively.³² Furthermore, patients with a <80% LSI for quadriceps strength have demonstrated greater asymmetry in knee kinematics and kinetics during landing than those with a >80% LSI for quadriceps strength.²⁶ Therefore, an LSI of 80.9% for quadriceps strength is considered a clinically reasonable cutoff value at 6 months after ACLR.

The ACL-RSI score at 12 months postoperatively was significantly different between the 2 groups, although the difference was not significant after adjusting for confounders including age, sex, and IKDC-SKF score at 6 months postoperatively. Previous studies have shown that younger age and male sex were associated with higher ACL-RSI scores.¹⁸ The relationship between psychological readiness and physical function after ACLR is still controversial. Some studies have shown a significant relationship between knee muscle strength and the ACL-RSI score,^22,34 while others have reported no such relationship.^1,25 Although cross-sectional studies have reported a moderate correlation between IKDC-SKF and ACL-RSI scores,^6,38 the present study suggests that better psychological readiness at 6 months postoperatively did not seem to affect improvement in subjective knee function from 6 to 12 months after ACLR after adjusting for age and sex. On the other hand, quadriceps strength at 3 months has been reported to be associated with the ACL-RSI score at 9 months.³⁴ In addition, an increase in quadriceps strength from preoperatively to the time of RTS was shown to be associated with the ACL-RSI score at the time of RTS.²² The present study also showed that improvement in the IKDC-SKF score from 6 to 12 months postoperatively was significantly correlated with improvement in the ACL-RSI score. Therefore, psychological readiness may not be associated with the recovery of physical function but may improve with the recovery of physical function.

The proportion of patients who underwent meniscectomy was significantly greater in the poor recovery group than in the good recovery group. In addition, logistic regression analysis showed that patients who underwent concomitant meniscectomy were 5.7 times more likely to have poor recovery of the IKDC-SKF score than those who did not undergo meniscectomy, even after adjusting for age, sex, and IKDC-SKF score at 6 months. These findings align with a previous study that reported that concomitant meniscal injuries diagnosed on magnetic resonance imaging were associated with low recovery of the IKDC-SKF score at 5 years after ACLR.²⁷ After adjusting for age, sex, and IKDC-SKF score at 6 months, the proportion of patients who underwent meniscal repair and meniscectomy was higher in the poor recovery group than in the good recovery group, but the results did not reach statistical significance, and statistical power may have been lacking. Future studies with a larger sample size should be performed to clarify the effects of concomitant meniscectomy and meniscal repair on improvement in subjective knee function from 6 to 12 months after ACLR.

The present study did not find an association between hamstring strength and improvement in the IKDC-SKF score from 6 to 12 months after ACLR. Therefore, the association between improvement in subjective knee function and hamstring strength would be weak compared with quadriceps strength. Previous cross-sectional studies have also shown an association of the IKDC-SKF score with quadriceps strength but not with hamstring strength.^8,32 On the other hand, a previous study reported that the LSI for hamstring strength at 3 months postoperatively was associated with the Lysholm score at 6 months.³⁰ Hamstring strength may be associated with subjective knee function within 6 months but not after 6 months postoperatively.

Limitations

There were some limitations. First, this retrospective study was a single-center study and had a small sample size. Different postoperative protocols and timing of RTS after ACLR may influence the results. External validity needs to be addressed in future studies. Second, this study included only patients who had undergone ACLR with a hamstring tendon autograft. It is known that there are differences in postoperative recovery between hamstring tendon and bone–patellar tendon–bone grafts, which may lead to different results. Third, the follow-up period of 12 months was relatively short. A time point of 12 months postoperatively is important in terms of short-term outcomes, such as RTS, and IKDC-SKF scores are stable,²⁷ but a longer follow-up is also needed.

Conclusion

The present study investigated the factors associated with improvement in the IKDC-SKF score from 6 to 12 months after ACLR. Greater asymmetry of quadriceps strength at 6 months postoperatively was associated with poor recovery of subjective knee function from 6 to 12 months, even after adjusting for age, sex, and IKDC-SKF score at 6 months postoperatively. Logistic regression analysis showed that the LSI for quadriceps strength at 6 months was associated with improvement in the IKDC-SKF score, with a cutoff value of 80.9%. The LSI for quadriceps strength at 6 months is important to improve the IKDC-SKF score from 6 to 12 months after ACLR.

Footnotes

Final revision submitted May 7, 2024; accepted June 20, 2024.

ORCID iDs

Tomoya Ishida

Makoto Suzuki

Mina Samukawa

Harukazu Tohyama

One or more of the authors has declared the following potential conflict of interest or source of funding: This work was supported by the Japan Society for the Promotion of Science KAKENHI (grant No. JP23K10553). T.I. and M. Samukawa were members of a joint research project with Nitto Denko, but the project is not related to the present study. M. Samukawa was also a member of a joint research project with Nitoms, but the project is not related to the present study. Work conducted by H.T. was funded in part by ORGO, but the project is not related to the present study. 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 Hokkaido University (No. 23-37-1).

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