Abstract
Background:
Sex-related differences in outcomes after adolescent anterior cruciate ligament reconstruction (ACLR) remain inadequately characterized in the literature. Preliminary studies suggest potential sex-based differences in postoperative complications, although findings regarding specific adverse outcomes remain inconsistent.
Purpose:
To evaluate sex-specific differences in postoperative complications after adolescent ACLR in a multicenter cohort with propensity matching and 2-year follow-up.
Study Design:
Cohort study; Level of evidence, 3.
Methods:
Data were retrospectively extracted from the TriNetX Research Network. Adolescents aged between 12 and 19 years who underwent ACLR were identified using Current Procedural Terminology codes. Male and female cohorts were propensity score-matched (1:1) for age, obesity, and concomitant meniscal repair. Outcomes at 3, 6, 12 months, and 2 years included subsequent ACLR revision, subsequent meniscal surgery, and other postoperative complications. Odds ratios with 95% CIs were calculated.
Results:
A total of 7793 adolescents per sex were propensity-matched. At 3 months, male adolescents had greater rehabilitation utilization (33.7% vs 31.1%; P = .003). At 6 months, female adolescents had higher manipulation under anesthesia (MUA), and lysis of adhesions (all, P < .001), whereas rehabilitation use remained higher in male adolescents (P = .02). At 1- and 2-year follow-up, female patients experienced higher rates of MUA and lysis of adhesions (all, P < .001). No differences were observed in subsequent ACLR, meniscal surgery, or any knee reoperation.
Conclusion:
Distinct postoperative complication profiles were observed between male and female adolescents after ACLR. Female patients consistently demonstrated a higher likelihood of postoperative stiffness requiring secondary procedures. In contrast, no sex-based differences were identified in subsequent meniscal surgery or revision ACLR. These findings highlight postoperative stiffness as a sex-specific complication in adolescents and underscore the importance of incorporating sex-based considerations into postoperative monitoring and future outcome-focused research.
Anterior cruciate ligament (ACL) tear is a common orthopaedic injury. In recent years, the incidence of these injuries has increased, specifically in the adolescent population. Rates of ACL tear in the teenage population are reported to be approximately 120 per 100,000 person-years, with peaks of 400 per 100,000 person-years in certain age groups. 5 Of these patients, 98% undergo ACL reconstruction (ACLR) within one year of injury.25,35 Despite the frequency of surgery, rehabilitation protocols following ACLR remain heterogeneous and often fail to account for patient-level modifiers such as sex. 25
Although female adolescents sustain anterior cruciate ligament (ACL) injuries more frequently than male adolescents,1,6,14,26,29,35 sex-related differences in outcomes after ACL reconstruction (ACLR) remain insufficiently characterized. Previous investigations have predominantly focused on adult cohorts and have emphasized functional outcomes—including patient-reported measures, strength testing, functional hop tests, and return-to-sports (RTS) timing.3,8,9,10,19,33,36,40 While these studies provide important insight into functional recovery, they offer limited information regarding clinically actionable postoperative events that may require additional intervention.
In contrast, data on sex-based differences in procedure-based clinical outcomes after ACLR in adolescents are sparse. Studies on adolescents have largely emphasized short-term functional recovery over long-term clinical outcomes. For example, Maguire et al 25 evaluated adolescents aged 12 to 19 years to compare injured and uninjured limbs in male and female adolescents. They found larger quadriceps strength deficits in female adolescents, with deficits in both sexes for hamstring strength, and no differences in dynamic balance or functional hop testing. That study has significant limitations—including a single-center design, a restriction to hamstring autograft, a short 6-month follow-up, and outcomes limited to functional testing.
Postoperative complications, such as arthrofibrosis requiring manipulation under anesthesia (MUA) or lysis of adhesions, represent clinically important adverse events that can delay recovery, increase health care utilization, and negatively impact long-term outcomes.13,15,16,31,34 Rehabilitation utilization reflects patterns of postoperative care delivery and may vary by sex, potentially influencing recovery trajectories. 4 In addition, subsequent surgical interventions, such as repeat meniscal surgery and subsequent or revision ACLR, are clinically relevant outcomes that may reflect structural failure, reinjury, or progression of intra-articular pathology. Although sex-based differences in these outcomes have been reported inconsistently in predominantly adult cohorts,8,9,19,33,36,40 corresponding data in adolescents remain limited.
Hence, the purpose of our study was to compare early and short-term postoperative outcomes after isolated primary ACLR in male and female adolescents using a large propensity-matched cohort with a 2-year follow-up.
Methods
Study Design
This retrospective cohort study was conducted using TriNetX, LLC (Cambridge). 24 This global health research network provided access to electronic medical records from approximately 100 million patients across large health care organizations (HCOs). On January 6, 2026, we queried the database on the Research network, which included 115 HCOs, to identify male and female adolescent patients who underwent primary ACLR. The primary objective was to compare postoperative complications between the male and female groups across early-, late-, and short-term follow-up periods. TriNetX is a Health Insurance Portability and Accountability Act-compliant database that provides real-time clinical data for de-identified patients; thus, the study was exempt from institutional review board approval.
Cohort Selection and Propensity Matching
Patients who underwent a primary ACLR between 2005 and 2023 were identified using Current Procedural Terminology code 29888. Eligible patients were male and female adolescents aged 12 to 19 years who underwent primary ACLR and had a minimum of 2-year follow-up at a TriNetX-affiliated facility. Patients with a history of ACLR, femoral or tibial osteotomy, or osteochondral transplantation were excluded. In addition, patients undergoing concomitant reconstruction of another major knee ligament—including the posterior cruciate ligament or the medial or lateral collateral ligament, at the time of the index ACLR were excluded to reduce heterogeneity related to injury severity and complex multiligament pathology.
Propensity score (1:1) matching was performed using the TriNetX algorithm to account for age, obesity, and concomitant meniscal repair at the time of the index ACLR. The platform integrates the nearest-neighbor matching with a tolerance level of .01 and ensures that the difference between propensity scores does not exceed P = .01. Covariate balance was assessed using standardized mean differences (SMDs), with an absolute SMD <0.10 considered indicative of adequate balance.
Outcome Measures
Ealy and late postoperative complications were evaluated as cumulative incidence from the index procedure within 3 and 6 months, and included subsequent meniscal surgery (meniscectomy or repair), MUA, subsequent ACLR, lysis of adhesions, surgical wound complications (superficial or deep surgical site infection, wound disruption, hematoma, or seroma), deep venous thromboembolism (DVT), emergency room (ER) visits, hospital readmission, and any knee reoperation (meniscal surgery or ACLR).
Short-term follow-up complications were evaluated at 1 and 2 years postoperatively and included subsequent meniscal surgery (meniscectomy or repair), MUA, ACLR, lysis of adhesions, and any knee reoperation (meniscal surgery or ACLR).
Specific codes used to define inclusion criteria and outcome variables are provided in the Appendix (Appendix Tables 1 and 2).
Data Analyses
Statistical analyses were performed using the TriNetX platform to ensure a robust evaluation of the data. To compare unmatched and matched cohorts, independent t tests were utilized for continuous variables, and chi-square analyses were applied for categorical variables. We assessed complication rates between male and female patients, calculating odds ratios (ORs) with 95% CIs for each comparison. Continuous variables were reported as mean values with standard deviations, and categorical variables were presented as absolute counts and percentages. A statistical significance threshold was set at P < .05.
To protect patient privacy per TriNetX policy, outcomes with <10 patients were not reported, and related percentages and ORs were withheld.
Results
The initial query identified 8683 male and 9581 female patients who had ACLR. After propensity-score matching, 7793 patients were included in each cohort for the final analysis (Figure 1). Balance was achieved across all matched covariates (age, obesity, and concomitant meniscal repair), with postmatching SMDs below the prespecified threshold. Demographic characteristics of both the unmatched and matched cohorts are presented in Table 1.
CONSORT flow of participants through the study. The asterisk indicates that no patients were lost to follow-up. ACLR, anterior cruciate ligament reconstruction; CONSORT, Consolidated Standards of Reporting Trials; PSM: Propensity score matching.
Demographic Characteristics Before and After Propensity-Score Matching Between Male and Female Adolescents Who Underwent ACLR Surgery a
Data are presented as mean + SD or n (%). Bold P values indicate statistical significance. ACLR, anterior cruciate ligament reconstruction; SMD, standardized mean difference.
Early and Late Postoperative Complications
At 3 months, male patients demonstrated greater utilization of rehabilitation services than female patients (33.1% vs 30.8%; OR, 1.11; P = .002). In contrast, the female cohort had higher odds of undergoing lysis of adhesions compared with the male group (OR, 4.2 [95% CI, 2.17-8.33]; P < .001).
At 6 months, female patients experienced significantly higher rates of MUA (0.7% vs 0.2%; OR, 0.22; P < .001) and lysis of adhesions (1.6% vs 0.4%; P < .001). Rehabilitation utilization remained significantly greater in male patients at 6 months (35% vs 32%; P = .002) (Table 2).
ACLR Postoperative Outcomes in Adolescent Male Versus Female Cohorts at 3-and 6-Month Follow-up a
Data are presented as n (%), unless otherwise indicated. Bold P values indicate statistical significance. ACLR, anterior cruciate ligament reconstruction; DVT, deep venous thromboembolism; ER, emergency room; OR, odds ratio.
No differences were observed in wound complications, DVT, ER visits, hospital readmissions, and subsequent meniscal or ACLR surgeries (Table 2).
Short-term Complications
At 1-year, female adolescents experienced higher rates of MUA (0.8% vs 0.2%; OR, 4.3 [95% CI, 2.44-7.69]; P < .001) and lysis of adhesions (2.2% vs 0.7%; OR, 3.1 [95% CI, 2.33-4.17]; P < .001). At 2 years, MUA and lysis of adhesions remained significantly higher in the female group (all, P < .001) (Table 3).
ACLR Postoperative Outcomes in Adolescent Male Versus Female Cohorts at 1- and 2-Year Follow-up a
Data are presented as n (%), unless otherwise indicated. Bold P values indicate statistical significance. ACLR, anterior cruciate ligament reconstruction; OR, odds ratio.
No differences were observed in subsequent meniscal surgery, subsequent ACLR, or any knee reoperation between the 2 groups (Table 3).
Discussion
In this large propensity-matched adolescent cohort, female adolescents demonstrated a consistently higher burden of postoperative stiffness requiring secondary procedures after ACLR. Specifically, female patients had higher rates of MUA and lysis of adhesions at early follow-up, and these differences persisted through 2 years. In contrast, no sex-based differences were observed in subsequent meniscal surgery, subsequent ACLR, or any knee reoperations at any timepoint. Early postoperative rehabilitation utilization was slightly higher in male patients, although absolute differences were modest.
The persistence of these differences in MUA and lysis of adhesions suggests a durable sex-associated susceptibility to arthrofibrosis rather than a transient postoperative phenomenon. Importantly, stiffness in this study was defined using objective, procedure-based endpoints rather than diagnosis codes, strengthening the clinical relevance of this finding. This is consistent with previously published literature, which describes the increased risk of arthrofibrosis in women after ACLR.13,15,16,31,34 A study by Murphy et al 27 investigated arthrofibrosis after ACLR in adolescents, with findings indicating that skeletal immaturity is an additional risk factor. Several proposed explanations for these sex-based differences have been put forward, encompassing both physiological and psychological factors. Avila et al 2 investigated inflammatory markers in synovial fluid at the time of ACLR, noting increased levels of the cytokines regulated on activation, normal T cell expressed and secreted, and basic fibroblast growth factor in patients who later required MUA/lysis of adhesions. It has been suggested that higher circulating estrogen and progesterone and lower circulating testosterone in women promote a more proinflammatory synovial environment after knee surgery. 30 Additionally, estrogen fluctuations have been shown to affect tendon and ligament laxity, which impacts the stability of the knee even after reconstruction.20,21,22,23,30,41,42
Male adolescents demonstrated slightly higher documented rehabilitation utilization at early follow-up, with differences observed at 3 and 6 months. However, the absolute differences between groups were small, and rehabilitation use was captured as a binary marker indicating the presence of at least 1 billed in-network physical therapy encounter, rather than the intensity, duration, or completeness of rehabilitation. As such, these findings should be interpreted cautiously, primarily as contextual information on early postoperative care patterns rather than as evidence of clinically meaningful differences. Rehabilitation use may also be underestimated in this cohort, as physical therapy performed outside participating health systems, school- or team-based athletic training, and home-based Rehabilitation programs are not captured in the database.
Despite the observed sex-based differences in stiffness-related interventions, no differences were identified between male and female adolescents in subsequent meniscal surgery, subsequent ACLR, or overall knee reoperation through 2 years of follow-up. This contrasts with findings from predominantly adult populations. Meniscal tear is a relatively common complication of ACLR, and male sex has previously been presented as an associated risk factor in the adult population.7,11,18,28,37,38 However, some studies found no sex-based differences.12,17 In addition, meta-analyses studies identify male sex as an independent predictor of revision surgery and graft rupture, with hazard ratios and ORs typically between 1.27 and 2 compared with women.32,39 The present adolescent cohort demonstrated no sex-based differences in subsequent meniscal surgery or revision ACLR, suggesting that risk patterns observed in the adult population may not directly translate to adolescents and that sex alone does not appear to confer a differential risk of structural failure or reoperation beyond stiffness-related complications.
This study benefits from a large, multicenter cohort, which enabled robust propensity-score matching and enhanced the generalizability of our findings. However, several limitations warrant consideration. The database did not include information on graft type, surgical techniques, injury mechanism, or sport participation, all of which could confound the observed associations. The timing from injury to reconstruction and the laterality of procedures were not available; accordingly, subsequent ACLR and meniscal surgery reflect any surgery after the index procedure and cannot distinguish ipsilateral from contralateral injury. Rehabilitation use was captured as a binary indicator of at least 1 billed in-network physical therapy encounter and does not reflect rehabilitation intensity, duration, adherence, or care received outside the database health systems. RTS timing and activity exposure were not available. As with all retrospective administrative database studies, coding inaccuracies, selection bias, unmeasured confounders, and missing or incomplete data may influence outcome classification and observed associations. Finally, the observational design precludes the establishment of causal relationships.
Conclusion
In this large propensity-matched cohort of adolescents undergoing isolated primary ACLR, female patients demonstrated a consistently higher risk of postoperative stiffness requiring secondary procedures—including MUA and lysis of adhesions. In contrast, no sex-based differences were observed in subsequent meniscal surgery, subsequent ACLR, or overall knee reoperation through 2 years of follow-up. These findings highlight postoperative stiffness as a sex-specific complication in adolescents and suggest that female patients may benefit from closer postoperative surveillance for early stiffness. Prospective studies incorporating detailed surgical variables, rehabilitation adherence, and functional outcomes are needed to further elucidate the mechanisms underlying sex-based differences in stiffness and to optimize postoperative management strategies.
Footnotes
Appendix
Representation of the Outcomes Definition: Medical Codes for Included Diagnoses and Procedures a
| Variable | Code |
|---|---|
| Any knee reoperation | CPT: 29880; CPT: 29881; CPT: 29882; CPT: 29883; CPT: 29888 |
| Meniscal surgery | CPT: 29880; CPT: 29881; CPT: 29882; CPT: 29883 |
| Manipulation under anesthesia | CPT: 27570 |
| Lysis of adhesions | CPT: 29884 |
| Hospital readmissions | CPT: 99232; CPT: 99233; CPT: 99231; CPT: 99235; CPT: 99234; CPT: 99236; HCPCS: G0316; HCPCS: G0379; SNOMED: 32485007; CPT:1013675 |
| ER visits | CPT: 99281; CPT: 99282; CPT: 99283; CPT: 99284; CPT: 99285; CPT: 1013723; CPT: 1013711; HCPCS: G0380; HCPCS: G0381; HCPCS: G0382; HCPCS: G0383; HCPCS: G0384 |
| Subsequent ACLR | CPT: 29888 |
| Surgical wound complications | ICD10CM: T81.41 ICD10CM: T81.41XA ICD10CM: T81.42 ICD10CM: T81.42XA ICD10CM: T81.31 ICD10CM: T81.31XA ICD10CM: T81.32 ICD10CM: T81.32XA ICD10CM: M96.84 ICD10CM: M96.840 ICD10CM: M96.842 |
| DVT | ICD10CM: I82.40 ICD10CM: I82.409 ICD10CM: I82.4Z ICD10CM: I82.4 ICD10CM: I82.43 ICD10CM: I82.402 ICD10CM: I82.49 ICD10CM: I82.401 |
| Rehabilitation utilization | CPT: 97161 CPT: 97164 CPT: 97110 CPT: 97112 CPT: 97116 CPT: 97140 CPT: 97530 CPT: 97750 |
ACLR, anterior cruciate ligament reconstruction; CPT, Current Procedural Terminology; DVT, deep vein thrombosis; ER, emergency room; HCPCS, Healthcare Common Procedure Coding System; SNOMED, Systematized Nomenclature of Medicine.
Final revision submitted January 12, 2026; accepted January 24, 2026.
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 was not sought for the present study.
