The Association of Tibia:Femur Ratio and Anterior Cruciate Ligament Injury

Abstract

Background:

The tibia:femur ratio (TFR) is an anatomic proportion describing the length of the tibia relative to the femur, with an established normative mean of 0.78 in skeletally mature individuals. Variation in TFR affects lower extremity biomechanics, and there is an association between elevated TFR and hip/knee osteoarthritis and patellar instability.

Hypothesis:

TFR variation may also be associated with anterior cruciate ligament (ACL) injury; thus, the purpose of this study is to determine if the mean TFR in patients with ACL injury is different from the mean normative TFR in the general population.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

A total of 50 patients with magnetic resonance imaging–confirmed ACL injury underwent full-length lower extremity scanograms. Femoral length was measured from the most superior point of the femoral head to the center point of the medial femoral condyle. Tibial length was measured from the center point of the medial tibial plateau to the center point of the tibial plafond. The TFR was calculated by dividing the tibial length by the femoral length. The mean TFR of the study group was compared with the previously published mean TFR for normative controls with a 2-tailed t test.

Results:

The mean TFR for the entire cohort of patients with ACL injury was 0.759 (SD, 0.029), which was significantly lower than the mean normative TFR of 0.781 (P = .0001). There were 29 (58.0%) male and 21 (42.0%) female patients. The mean TFR was similar for male (0.760) and female (0.757) patients (P = .77).

Conclusion:

Patients with ACL injury demonstrated a significantly lower TFR than the previously published TFR for normative controls, which suggests that an association between TFR and ACL injury may exist. It is possible that decreasing tibial length relative to femoral length alters lower extremity biomechanics in such a manner that places the ACL at risk for injury.

Several intrinsic risk factors for anterior cruciate ligament (ACL) injury are known to exist, including younger age, female sex, increased body mass index (BMI), and anatomic variations such as decreased notch width index, increased posterior tibial slope, and increased ligamentous laxity.^1,3,16,21 Furthermore, neuromuscular imbalances that result in altered lower extremity biomechanics during jump-landing and pivoting activities can place undue strain on the ACL and increase the risk of injury.^6,14 Alterations in jump-landing mechanics that place the ACL at risk for injury include a knee valgus or abduction moment, increased hip flexion, stiff knee and hip, and center of mass (COM) posterior to the base of support at the time of landing.^4,10,18 Knee abduction moment is influenced by tibial length, and it is a known risk factor for ACL injury and patellofemoral pain syndrome.^11,12

The tibia:femur ratio (TFR), also known as the crural index, is an anatomic proportion that describes the length of the tibia relative to the length of the femur, with an established mean normative value of approximately 0.78 in skeletally mature individuals based on full-length scanogram radiographs.^2,7 Variation of the TFR produces subtle changes in lower extremity biomechanics that may influence gait, run speed, jump height, and squat mechanics.^8,17,20 It is thought that lower extremity relative limb length may have an effect on the squatting mechanics that athletes adopt, as athletes with relatively longer femurs demonstrate increased trunk and hip flexion during a squat.^9,13,19 Additionally, several authors have identified an association between increased TFR and pathologic states such as hip and knee osteoarthritis and patellar instability.^22,23 However, it is currently unknown if variation of the TFR is also associated with risk of ACL injury. The purpose of this study is to determine if the TFR in patients with ACL injury differs from the mean normative TFR in the general population.

Methods

This prospective study protocol was approved by the institutional review board of St Luke's University Health Network, and informed consent was provided by all participating patients or their legal guardians. A total of 50 patients diagnosed with acute ACL injury who were evaluated at our orthopaedic sports medicine clinic from July 1, 2023, to September 1, 2024, were enrolled in this study. Inclusion criteria were (1) acute knee injury with positive Lachman or pivot shift on examination, (2) magnetic resonance imaging (MRI) confirmation of ACL tear, and (3) skeletally mature patients age ≥14 years. Exclusion criteria included (1) recurrent ACL injuries with history of previous reconstruction, (2) multiligamentous knee injuries or other pathologic knee states such as arthritis, (3) lack of MRI confirming ACL tear, and (4) skeletally immature patients or age <14 years. All patients meeting inclusion criteria were informed of the study and asked to provide consent to participate without any selection bias.

For each study participant, a full-length lower extremity scanogram was obtained and utilized to determine the TFR of the injured limb. Femoral length was measured from the most superior point of the femoral head with a straight line to the center point of the medial femoral condyle. Tibial length was measured from the center point of the medial tibial plateau with a straight line to the center point of the tibial plafond. This method is the same as described in the Aitken ² study for measurement of the TFR. The TFR was calculated by dividing the tibial length by the femoral length (Figure 1). Each measurement was recorded by 2 observers, 1 orthopaedic surgery resident physician (J.M.) and 1 board-certified radiologist. The mean TFR of the study group was compared with the mean normative TFR published in a study using the same measurement technique. ²

Figure 1.

Full-length scanogram (femoral and tibial radiographs). Femoral length was measured from the most superior point of the femoral head with a straight line to the center point of the medial femoral condyle. Tibial length was measured from the center point of the medial tibial plateau with a straight line to the center point of the tibial plafond.

Statistical Analysis

Statistical analysis was performed using a 2-tailed Student t test. Interobserver reliability was assessed using 2-way mixed intraclass correlation coefficients. Significance was set to a P value <.05.

Results

The mean TFR for the entire cohort of patients with ACL injury (N = 50) was 0.759 (SD, 0.029), which was significantly lower than the mean normative TFR of 0.781 (n=1077 (SD, 0.04) (P = .0001). The study cohort consisted of 29 (58.0%) male and 21 (42.0%) female patients. The mean age of the cohort was 26.1 years. The mean BMI was 26.0. Race reported by the patients was 78% White, 16% Black, 2% mixed, and 4% other. Ethnicity reported by the patients was 82% non-Hispanic, 16% Hispanic, and 2% other. When the cohort was analyzed by both race and ethnicity, the reported percentages were 70% White non-Hispanic, 12% Black non-Hispanic, 8% White Hispanic, 4% Black Hispanic, 4% other Hispanic, and 2% mixed other. The mean TFR was similar for male (0.760 [SD, 0.031]) and female (0.757 [SD, 0.027]) patients (P = .77). The intraclass correlation coefficient was 0.89, indicating good interrater reliability.

Discussion

Patients with ACL injuries in this cohort demonstrated significantly lower tibia:femur ratios than the normative tibia:femur ratio. There was no difference in TFR between male and female patients for this cohort. This study also demonstrated good interrater reliability when evaluating the TFR on full-length scanograms.

Previous research has demonstrated that athletes may exhibit different postures when performing movements such as the back squat and standing countermovement jump.^8,17 Athletes may assume a posture of more knee flexion or more hip flexion to achieve similar depths in these movements. Greater knee flexion typically results in a more upright trunk position, whereas more hip flexion results in more trunk flexion and less knee flexion. ¹⁹ Demers et al ⁸ observed that athletes with longer femurs relative to their tibias tended to flex their trunk and hips more and widen their stance in the back squat compared with athletes with shorter femurs. They also found that increased thigh:shank ratio (decreased tibia:femur ratio) resulted in the COM being shifted posterior in a squatting exercise. Widening the stance and utilizing more trunk and hip flexion with less knee flexion during squatting causes the COM to shift posterior to the base of support, and it is associated with increased length of the femur relative to the tibia. This shift in COM may be small, similar to previous findings on anteroposterior shifts in center of pressure (COP) of the foot in patients undergoing rehabilitation for ACL reconstruction. ⁵ Chan and Sigward ⁵ found that more anterior COP was associated with more demand at the hip and ankle than the knee compared with posterior COP and was a compensatory pattern in patients undergoing ACL rehabilitation.

Landing with the COM posterior to the base of support is also known to be a risk factor for ACL injury. ¹⁸ It is possible that a relatively long femoral length compared with the tibia predisposes movement toward COM posterior to the base of support in squatting in landing movements. It has also been documented that less knee flexion upon landing is a risk factor for ACL injury, which may be exhibited in athletes who adopt a COM posterior squat or landing style.^6,14,15 Anatomic variation in tibia:femur ratio likely has an effect on the movement patterns of athletes, and it is possible that athletes exhibiting COM posterior movement and less knee flexion are at a higher risk for ACL injury.

This study demonstrated a significantly lower tibia:femur ratio in ACL-injured patients when compared with normative values of a historical control. It is possible that athletes with longer femurs relative to their tibia may exhibit movement patterns that involve less knee flexion and COM posterior to the base of support, placing them at risk for ACL injury. This study only establishes an association of tibia:femur ratio and ACL injury. Although female sex is known to be a risk factor for ACL injury, there were no differences in tibia:femur ratio between men and women in this study; therefore, it appears that this association is independent of sex.

Limitations

The major limitation of this study is the lack of a control group of individuals without ACL injury. A historical control using a previously published mean normative value was thought to be reasonable for the purpose of this initial investigation of TFR and ACL injury. The historical control demographic was from a study performed in the Netherlands that was 61% male and had a mean age of 51.2 years compared with the ACL tear demographic of the current study that was 58.0% male and had a mean age of 26.1 years.

This is the first study to evaluate the relationship between TFR and ACL injury. Patients with ACL injury were found to have a significantly decreased TFR compared with the general population. These findings suggest that additional studies are warranted to understand TFR variation more clearly as a risk factor for ACL injury. Additional research could be directed with establishment of a control group without ACL injury and those with ACL injury for comparison. Moreover, it is unclear if TFR variation has a definitive causative effect on the adoption of compensatory movement patterns in squatting and jumping, and further research could be directed at better delineation of these differences, if they exist. With better understanding of this association, surgeons may be able to counsel patients on risk factors of ACL injury as well as assist surgeons in developing prevention strategies before and after ACL injury. Most importantly, this study establishes a basis for this measurement and will allow for more examination of the TFR relationship to ACL injury.

Conclusion

This cohort of patients with ACL injury demonstrated a significantly lower TFR than normative TFR in a cohort of skeletally mature individuals, which suggests that an association between TFR and ACL injury may exist. It is possible that decreasing tibial length relative to femoral length alters lower extremity biomechanics in such a manner that places the ACL at risk for injury. Further investigation is warranted to better understand the relationship between TFR and the risk of ACL injury.