Early Patellofemoral Underloading After ACL Reconstruction Predicts Worse Cartilage: What This Means for Post‑Traumatic Knee Osteoarthritis

Highlights

– Reduced patellofemoral contact force measured within 1 year after anterior cruciate ligament reconstruction (ACLR) is associated with worse trochlear cartilage structure 1–5 years later (pooled r = -0.48, 95% CI -0.63 to -0.31).

– No consistent relationship was found between early knee joint kinetics (e.g., knee flexion moment, knee adduction moment) and future tibiofemoral structural outcomes in pooled analyses.

– Lower frontal plane kinetic measures appear to be associated with better future symptoms, suggesting a complex relationship between mechanical loading and clinical outcomes.

Background

Post‑traumatic osteoarthritis (PTOA) of the knee is a major clinical problem after traumatic knee injury. Up to one in two individuals develop symptomatic or structural knee OA within a decade of an injury such as anterior cruciate ligament (ACL) rupture. In primary (insidious) knee OA in older adults, altered biomechanics—particularly increased medial tibiofemoral loading reflected by a higher knee adduction moment—have been linked to disease onset and progression. Whether similar biomechanical markers early after traumatic knee injury predict future PTOA has been less clear. Understanding these relationships is clinically important because biomechanics are potentially modifiable by rehabilitation, orthoses, gait retraining and surgical techniques.

Study design

Scope and selection

This systematic review and meta‑analysis (PROSPERO CRD42024504099) synthesised longitudinal studies that enrolled participants after surgical treatment for traumatic knee injury, quantified dynamic knee biomechanics (kinetics, kinematics) during tasks such as walking or hopping, and reported associations between early biomechanics and later structural (imaging) or symptomatic outcomes.

Included studies

Eighteen longitudinal studies were included: 17 studies following ACL reconstruction and one study after meniscectomy. Imaging endpoints were predominantly MRI outcomes (T2 or T1ρ relaxation times, cartilage defects), with a smaller number using radiographs; symptomatic outcomes used validated patient‑reported measures. Meta‑analysis was performed when at least three comparable studies reported the same biomechanical exposure and an outcome that could be pooled; otherwise findings were synthesised narratively.

Key findings

Patellofemoral joint

The most consistent signal emerging from pooled data concerned patellofemoral joint loading. Meta‑analysis of three studies evaluating patellofemoral contact force measured within the first postoperative year (up to 1 year) and MRI‑assessed cartilage outcomes 1–5 years later found that lower patellofemoral contact force was associated with worse trochlear cartilage structure (pooled correlation r = -0.48, 95% CI -0.63 to -0.31; I2 = 0%). This effect size is moderate and suggests a meaningful inverse relationship: the less the patellofemoral joint is loaded early after ACLR, the worse the trochlear cartilage appears on subsequent MRI.

The pooled relationship with patellar cartilage was not statistically significant (pooled r = -0.09, 95% CI -0.30 to 0.12; I2 = 0%), indicating a probable anatomical or loading‑pattern specificity to the observed effect. Narrative synthesis supported the trochlear finding: several studies reported associations between lower patellofemoral kinetics and worse trochlear cartilage metrics (elevated T2 values or defect progression), whereas findings for the patellar cartilage were inconsistent.

Tibiofemoral joint

A meta‑analysis of three studies examining early joint kinetics (e.g., external knee flexion moment, knee adduction moment) and later tibiofemoral structural outcomes (T1ρ values, cartilage defects on MRI, radiographic OA) found no significant pooled associations. Heterogeneity in biomechanical metrics, timepoints, outcome measures and patient characteristics limited definitive conclusions. Narratively, some studies indicated that lower kinetic measures were linked to worse trochlear cartilage (again implicating the patellofemoral compartment), whereas relationships with tibiofemoral structure were inconsistent—contrasting with the established link between greater frontal plane loading and medial tibiofemoral OA progression in insidious OA.

Symptoms

Findings for patient‑reported symptoms were heterogeneous but revealed an intriguing pattern: lower frontal plane kinetics (e.g., knee adduction moment or medial ground reaction force) were associated with better future symptoms, independent of the timepoint. Time since surgery moderated many associations between mechanical loading and symptoms—early postoperative biomechanics may reflect pain‑avoidance or neuromuscular inhibition, while later biomechanics reflect adaptation and recovery. The apparent divergence—unloading associated with worse structural patellofemoral outcomes but lower frontal plane kinetics associated with better symptoms—highlights the complex and sometimes dissociated relationships between structure, loading and clinical symptoms after knee injury.

Magnitude and precision of effects

The primary pooled effect (patellofemoral contact force → trochlear cartilage) was moderate (r ≈ -0.48) with narrow confidence intervals and no detectable between‑study heterogeneity (I2 = 0%). Other pooled analyses were null or underpowered, with inconsistent directional findings and substantial methodological heterogeneity that precluded robust pooled estimates for many biomechanical–outcome pairs.

Expert commentary and interpretation

These findings suggest that underloading of the patellofemoral joint early after ACLR may be a risk marker for later deterioration of trochlear cartilage. Mechanistically, cartilage health depends on optimal cyclic loading; insufficient load may impair chondrocyte mechanotransduction, matrix turnover and nutrition, promoting degenerative changes. After ACL injury and reconstruction, quadriceps weakness, pain, and altered movement strategies commonly reduce patellofemoral contact forces—potentially initiating or accelerating cartilage degeneration in the trochlea.

Notably, this pattern differs from classical medial tibiofemoral OA models where excessive frontal plane loading is detrimental. The absence of clear associations between early tibiofemoral kinetics and later tibiofemoral degeneration in this review could reflect several factors: younger cohorts with different loading patterns, the prominence of concurrent meniscal or chondral injury as dominant drivers of tibiofemoral PTOA irrespective of gait mechanics, limited follow‑up duration to detect tibiofemoral radiographic changes, or inadequate sensitivity of early biomechanical markers to capture relevant tissue stress in the tibiofemoral compartment.

From a clinical standpoint, these data support proactive rehabilitation strategies that safely restore patellofemoral loading—targeting progressive quadriceps strengthening, functional loading exercises, and task‑specific gait or landing retraining—while monitoring for pain and signs of persistent avoidance. However, trial evidence that altering early biomechanics prevents PTOA is lacking; the current findings are associative.

Limitations

Important limitations temper the conclusions. First, most included studies were conducted in ACLR populations, limiting generalisability to other injury types. Second, sample sizes were modest and follow‑up typically short to medium term (1–5 years); many PTOA processes unfold over longer periods. Third, biomechanical exposures varied across studies (different tasks, modelling approaches, and contact force estimations), creating heterogeneity. Fourth, imaging endpoints (T2/T1ρ) are sensitive to compositional cartilage changes but are surrogate markers rather than clinical OA end‑points. Finally, confounding by factors such as meniscal status, chondral lesion severity, graft choice, activity level, and rehabilitation intensity was variably accounted for across studies.

Clinical implications and practical recommendations

– Assess patellofemoral loading and quadriceps function during early rehabilitation after ACLR. Objective measures (isometric/quadriceps strength, functional hop tests) and, where available, instrumented gait analysis can help identify underloading patterns.

– Design rehabilitation to progressively restore patellofemoral loading: graded quadriceps strengthening, closed‑chain strengthening, progressive plyometrics and controlled return‑to‑sport drills. Emphasise pain‑contingent but progressive exposure rather than chronic avoidance.

– Be cautious when interpreting frontal plane kinetics: although lower frontal plane loading related to better symptoms in several studies, its relationship to long‑term structure is unclear in PTOA and differs from patterns in degenerative OA.

– Consider monitoring at‑risk patients (persistent quadriceps weakness, persistent avoidance of knee flexion/loading) with serial clinical assessments and, where indicated, targeted MRI surveillance.

Research gaps and future directions

There is a need for larger, harmonised prospective cohorts with standardised biomechanical assessments, well‑characterised injury and surgical variables, and long‑term clinical and structural outcomes. Future studies should:

• Standardise biomechanical exposure metrics (task, modelling, contact force estimation) to improve comparability and enable pooled analyses.
• Integrate muscle strength, neuromuscular control, and activity monitoring to better understand causal pathways from altered mechanics to cartilage degeneration.
• Test whether targeted interventions that restore patellofemoral loading (randomised controlled trials of rehabilitation or gait retraining) reduce the incidence or progression of PTOA.
• Examine a broader range of injuries (meniscal, multi‑ligament) and diverse populations to enhance external validity.

Conclusion

Current longitudinal evidence indicates that reduced patellofemoral contact force in the first year after ACL reconstruction is associated with worse trochlear cartilage structure 1–5 years later, while evidence linking early tibiofemoral kinetics to later tibiofemoral degeneration is inconsistent. Clinicians should be aware that early underloading of the patellofemoral joint may represent a modifiable risk marker for PTOA, and rehabilitation strategies that safely restore patellofemoral loading warrant priority. Definitive trials are needed to determine whether correcting biomechanical deficits prevents long‑term structural and symptomatic PTOA.

Funding and registration

Protocol registration: PROSPERO CRD42024504099. Primary review article: Savage M, Culvenor AG, Hedger M, Matt AR, O’Brien MJM, McMillan RM, De Livera A, Mentiplay BF. Are Altered Knee Joint Biomechanics Associated with Future Post‑Traumatic Osteoarthritis Outcomes? A Systematic Review and Meta‑Analysis of Longitudinal Studies. Sports Med. 2025 Oct;55(10):2595‑2612. doi: 10.1007/s40279‑025‑02288‑1.

References

Primary systematic review cited above (Savage et al., Sports Med. 2025). Additional background references are available in that review.