Abstract
Background:
Lateral tibiofemoral articular geometry may play a role in the development of non-contact anterior cruciate ligament (ACL) injuries. We hypothesized that athletes who had sustained an ACL injury would demonstrate more highly convex articular surfaces in the lateral compartment of the knee compared with activity-matched athletes who had not sustained an ACL injury, and that women would demonstrate greater absolute and relative convexity of these articular surfaces than men.
Methods:
One hundred and twelve athletes with a non-contact ACL injury and sixty-one activity-matched athletes without an ACL injury were studied. Three blinded observers measured the articular geometry in the mid-lateral sagittal plane with use of magnetic resonance imaging. The tibial plateau radius of curvature (TPr), distal femoral radius of curvature (Fr), maximal femoral anteroposterior articular length (FAP), and maximal tibial anteroposterior articular length (TPAP) were recorded. The Fr:TPr and FAP:TPAP ratios were also calculated to adjust for size variations. The intraclass correlation coefficient and the two-sample Student t test were used to compare quantitative variables. All data were found to follow a normal distribution.
Results:
When data for male and female patients were combined, the mean TPr, Fr, and TPAP values were significantly smaller in the ACL-injured patients than in the uninjured patients (33.9 compared with 37.5 mm, p = 0.005; 24.3 compared with 25.1 mm, p = 0.04; and 31.5 compared with 33.1 mm, p = 0.007; respectively). The mean FAP value did not differ significantly between the ACL-injured and uninjured patients but the difference in the mean FAP:TPAP value was significant (p = 0.003). When only male patients were analyzed, the mean TPr, Fr, and TPAP values were also significantly smaller in the ACL-injured patients than in the uninjured patients (35.5 compared with 41.1 mm, p = 0.002; 25.5 compared with 26.7 mm, p = 0.001; and 33.0 compared with 35.5 mm, p = 0.0002; respectively). The mean FAP value did not differ significantly between the ACL-injured and uninjured male patients, but the difference in the mean FAP:TPAP value was significant (p = 0.0005). In contrast, when only female patients were analyzed, none of the mean values differed significantly between the ACL-injured and uninjured patients. The FAP:TPAP and Fr:TPr values did not differ significantly among the ACL-injured male patients, injured female patients, and uninjured female patients.
Conclusions:
All female patients (both ACL-injured and uninjured) and ACL-injured male patients shared a common lateral knee geometry characterized by a smaller tibial plateau length relative to the femur and by more convex articulating surfaces of the proximal aspect of the tibia and the distal aspect of the femur. Shorter, more highly convex articulating surfaces may be inherently less stable with regard to anterior tibial translation and rotation. These findings may partially explain the greater overall predisposition of women compared with men toward ACL injury as well as why some studies have demonstrated no sex differences in graft reinjury after ACL reconstruction.
Level of Evidence:
Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.
Anterior cruciate ligament (ACL) ruptures are prevalent in many societies, and the resulting cost in the U.S. has been estimated to be as much as one billion dollars annually1. The pathomechanics that lead to rupture of the ACL remain controversial2-11. It is accepted that ACL injuries are influenced by multiple factors. Extrinsic factors include the sports playing surface, shoe wear, contact with other athletes, and weather. Intrinsic factors include developmental, kinematic/biomechanical, neuromuscular, and anatomic factors. Only modest success in preventing ACL injuries has been reported6,12. ACL injury appears to be an endemic problem among women, whose risk of non-contact ACL injury is two to five times that of men6,7,11,13,14. The high incidence of these injuries and the male-female discrepancy in incidence have driven research into understanding the anatomic factors associated with ACL tears.
Previous investigators have explored multiple anatomic factors including the intercondylar notch width15-18, the ACL size19, ligamentous laxity20,21, and the Q angle22. Biomechanical and clinical studies have indicated an association between ACL injury and increased posterior slope of the lateral proximal aspect of the tibia23-27. Hashemi et al. noted male-female differences in the slopes of the medial and lateral proximal aspects of the tibia in both the sagittal and the coronal plane24 and identified these slopes as potential risk factors for ACL injury28.
An association between the surface geometry of the tibial plateau and clinical instability in ACL-deficient knees has been hypothesized by Kujala et al.29 and Matsumoto9. Previous studies on the anatomy of this surface have focused on the relative length, width, concavity, or depth of the tibial plateau surface as measured with use of radiographs, computed tomography (CT), or magnetic resonance imaging (MRI), and the subchondral bone was taken to represent the articular surface9,24,27-31. To our knowledge, no study has quantitatively assessed the association between the sagittal-plane length and curvature of the articulating chondral surface and ACL rupture.
Theoretically, an eccentric axial load across opposing convex surfaces will result in translational and/or rotational instability. Thus, increased convexity of the distal aspect of the lateral femoral condyle and/or the lateral aspect of the tibial plateau could result in greater knee instability compared with flatter bearing surfaces.
We hypothesized that an analysis of the morphology of the lateral compartment of the knee would demonstrate the articular surfaces of the lateral aspect of the tibial plateau and/or the distal aspect of the femur to be more highly convex in patients with an ACL injury compared with patients of similar age and activity level without an ACL injury. We further hypothesized that female athletes would demonstrate greater absolute and relative articular convexity in the lateral compartment of the knee joint, which could help to explain the female predisposition toward ACL injury.
A chart review identified all patients who had presented to our institution's sports medicine clinic for a non-contact knee injury related to athletics between 2004 and 2010. Inclusion criteria were an age of sixteen to forty-five years, the availability of knee MRIs of suitable quality, and a score of 6 or greater on the Tegner activity questionnaire32 (indicating regular participation in sports involving cutting movements, such as tennis, badminton, handball, basketball, skiing, or jogging five times per week). Exclusion criteria were evidence of osteoarthritis on radiographs or MRIs and meniscal or other intra-articular pathologic injury that precluded measurement of chondral anatomy. The patients who satisfied these criteria were classified according to the presence of ACL injuries.
Twenty-five female and thirty-six male patients were classified as having no ACL injury. The MRIs acquired after the knee injury demonstrated no evidence of partial or complete ACL disruption, and the patients had no history of knee instability, of prior ipsilateral or contralateral ACL injury, or of prior surgery involving the medial or lateral meniscus.
Twenty-six female and forty-three male patients were classified as having a unilateral ACL injury. The MRIs acquired after the knee injury demonstrated an isolated ACL rupture or an ACL rupture with a concomitant tear of the medial meniscus, and these patients underwent ACL reconstruction. These patients had no previous history of ipsilateral or contralateral ACL injury or knee surgery.
Twenty-one female and twenty-two male patients were classified as having a bilateral ACL injury. The MRIs acquired after the knee injury demonstrated an isolated ACL rupture with or without a tear of the medial meniscus, and these patients underwent ACL reconstruction. These patients also had a clinical history of a previous non-contact ACL injury in the contralateral knee, returned to athletics after successful reconstruction of the ACL in that knee, and subsequently sustained the non-contact injury of the ACL in the other knee.
Demographic and injury data were recorded. Each series of T2-weighted proton-density MRIs was prescreened by the senior author (C.J.W.) to ensure that the axial, coronal, and sagittal reconstructed images were orthogonal to the posterior femoral condyles and that the articular surfaces could be accurately and precisely visualized. An image series was excluded if its utility was limited by motion artifact, low magnetic field strength or poor resolution, blooming artifact due to a retained metallic implant (e.g., an intramedullary nail), osteoarthritis, osteochondral defects, or a previous fracture. Acceptable MRIs were transferred in DICOM format from our institution's electronic PACS (picture archiving and communication system) software (Centricity; GE Healthcare, Waukesha, Wisconsin) to OrisiX imaging software (version 3.6.1, OsiriX Foundation, Geneva, Switzerland), which was used to perform all measurements and digital analysis. Each file was keyed to a study number, and identifying patient information (name, date, age, sex, diagnosis) was removed from the MRIs so that the observers who performed the measurements were blinded to patient sex and age. However, because some images demonstrated increased signal intensity (a bone bruise pattern) that suggested the presence of an ACL tear, the observers could not be completely blinded to the injury group.
A detailed description of the methods used to measure the maximal anteroposterior length of the femur (FAP), maximal anteroposterior length of the tibial plateau (TPAP), articular radius of curvature of the lateral aspect of the tibial plateau (TPr), and (Fibonacci) radius of curvature of the distal aspect of the femur (Fr) as well as the FAP:TPAP and Fr:TPr ratios is given in the Appendix.
The longitudinal axis of the femur was determined on a series of sagittal T2-weighted proton-density MRIs as described by Hashemi et al.24,28 so that its perpendicular could later be used to identify the maximal anteroposterior length of the femur (FAP).
Linked, cross-referenced coronal and sagittal-plane images were then used to identify the mid-lateral plane of the tibiofemoral articulation. This plane was defined as the sagittal-plane image slice that passed through the most caudad point on the distal femoral articular cartilage surface (the point corresponding to the mediolateral midpoint of the femoral and tibial articular surfaces). This sagittal reference plane was agreed on by the three observers and was used for all measurements in this study (Fig. 1).
Measurement of the Femoral Anteroposterior Length (FAP)
The anteroposterior length of the lateral femoral condyle (FAP) was defined as the length of the longest line connecting the anterior and posterior articular cartilage surfaces that could be drawn perpendicular to the anatomic axis of the femur (see Appendix).
Measurement of the Tibial Plateau Anteroposterior Length (TPAP)
The anteroposterior length of the lateral aspect of the tibial plateau (TPAP) was defined as the length of the longest line connecting the anterior and posterior articular cartilage surfaces that could be drawn within the subchondral plate of the tibial plateau (see Appendix). Since it was assumed that, on average, male patients would have larger knees than female patients, the ratio of anteroposterior femoral length to tibial length (FAP:TPAP) was also calculated to assess whether the size of the tibia relative to that of the femur was associated with ACL injury regardless of patient size.
Measurement of the Tibial Plateau Radius of Curvature (TPr)
The radius of curvature of the articular cartilage surface of the lateral aspect of the tibial plateau (TPr) was determined by digitally superimposing a circle on the articular surface of the tibial plateau (see Appendix); this method is analogous to that of Baré et al.33.
Measurement of the Fibonacci Femoral Radius of Curvature (Fr)
Because the shape of the distal aspect of the femur does not lend itself to superscription with a circle, we developed a novel measurement method based on the observation that the shape of the distal aspect of the femur follows a logarithmic curve with a diameter that increases from posterior to anterior. Such spiral curves are omnipresent in nature and frequently approximate the “golden ratio” of 1:1.618 described by the mathematician Fibonacci in 1202 (see Appendix). We found that the morphology of the distal aspect of the femur could be more easily approximated with a Fibonacci spiral than with a circle superimposed on the posterior condyle. The Fibonacci radius of femoral curvature (Fr) was defined as the length of the radial segment of the Fibonacci series at the point of contact between the femur and the tibia (see Appendix).
The ratio of the radius of curvature of the tibial plateau to that of the distal aspect of the femur (Fr:TPr) was also calculated to control for size differences due to sex and to variations among individuals.
Statistical Analysis
Statistical analysis was performed with use of SAS software (version 9.2; SAS Institute, Cary, North Carolina). A p value of <0.05 (two-tailed) was considered significant. All measurements were performed by three independent observers, and intraclass correlation coefficients (ICCs) were calculated to verify agreement among the observers. The two-sample Student t test was used to determine whether the previously described measurements differed significantly among the three study cohorts. All data were found to follow a normal distribution.
Source of Funding
No external funding source was employed in this investigation.
Patient Demographics
The distribution of patient ages, injury mechanisms, and Tegner activity scores is summarized in the Appendix. The age distribution of the male and the female patients differed; 53% of the male patients were twenty to twenty-nine years of age, whereas the female patients were more evenly distributed across all of the age groups (p = 0.004). Fifty-five percent of the injuries occurred during participation in soccer, basketball, or skiing. The Tegner activity score was 6 in sixty-six patients (38%), 7 in sixty-five (38%), 8 in seven (4%), 9 in thirty-four (20%), and 10 in one (1%). The groups with no, unilateral, and bilateral ACL injury did not differ significantly with regard to the Tegner score, but the difference between male and female patients was significant (p = 0.04).
Intraclass Correlation Coefficients (ICCs)
Reliability data for the four directly measured variables are given in the Appendix. The ICC between observers was classified as almost perfect for measurements of femoral anteroposterior length (0.90), Fibonacci femoral radius of curvature (0.90), and lateral tibial plateau radius of curvature (0.86), and moderate for measurements of the tibial plateau anteroposterior length (0.57).
Lateral Geometry Measurements
Differences According to Sex in the Entire Study Cohort (Table I)
When ACL-injured and uninjured patients were combined, significant differences between female and male patients existed with regard to the femoral anteroposterior length (FAP) (p < 0.001), lateral tibial plateau anteroposterior length (TPAP) (p < 0.001), femoral radius of curvature (Fr) (p < 0.001), lateral tibial plateau radius of curvature (TPr) (p < 0.001), and lateral femoral-to-tibial anteroposterior length ratio (FAP:TPAP) (p = 0.01). No significant difference existed with regard to the femoral-to-tibial radius ratio (Fr:TPr) (p = 0.56).
Differences According to the Presence of an ACL Injury in the Entire Study Cohort (Table II)
When male and female patients were combined, no significant differences between ACL-injured and uninjured patients existed with regard to the femoral anteroposterior length (p = 0.31). However, the injury groups differed significantly with regard to the tibial anteroposterior length (p = 0.007), femoral radius of curvature (p = 0.04), tibial radius of curvature (p = 0.005), length ratio (p = 0.003), and radius ratio (p = 0.03).
Differences According to Sex in Patients without an ACL Injury (Table III)
Male patients had significantly greater mean femoral anteroposterior length, tibial anteroposterior length, femoral radius of curvature, and tibial plateau radius of curvature. Male and female patients also differed with regard to the mean length ratio and radius ratio, although only the difference in the length ratio reached significance.
Differences According to the Presence of an ACL Injury in Female Patients (Table IV)
No significant differences between ACL-injured and uninjured female patients existed with regard to the mean femoral anteroposterior length (p = 0.65), tibial anteroposterior length (p = 0.77), length ratio (p = 0.75), femoral radius of curvature (p = 0.83), tibial radius of curvature (p = 0.68), and radius ratio (p = 0.80).
Differences According to the Presence of an ACL Injury in Male Patients (Table V)
No significant differences between ACL-injured and uninjured male patients existed with regard to the mean femoral anteroposterior length (p = 0.23). However, ACL-injured male patients had a significantly smaller mean tibial anteroposterior length compared with uninjured male patients (33.0 compared with 35.5 mm, p < 0.001). The mean femoral-to-tibial anteroposterior length ratio was significantly greater in ACL-injured male patients (2.20) than in uninjured male patients (2.05, p < 0.001). The femoral radius of curvature was smaller in ACL-injured male patients (25.5 mm) than in uninjured male patients (26.7 mm, p = 0.001). A significant and more substantial difference was noted in the mean tibial plateau radius of curvature, with ACL-injured male patients having a smaller radius (35.5 mm) compared with uninjured male patients (41.1 mm, p = 0.002).
Differences According to Sex in ACL-Injured Patients (Table VI) and a Comparison with Uninjured Female Patients
Significant differences between ACL-injured male and female patients existed with regard to the femoral anteroposterior length (p < 0.001), tibial anteroposterior length (p < 0.001), femoral radius of curvature (p < 0.001), and tibial radius of curvature (p = 0.004). However, when we attempted to control for sex-related size variations with use of ratios, we observed no significant difference between ACL-injured male and female patients with regard to the femoral-to-tibial length ratio (2.20 compared with 2.22, p = 0.48) or radius ratio (0.75 compared with 0.74, p = 0.65). We also compared the ratios of ACL-injured male patients and uninjured female patients; neither the femoral-to-tibial length ratio (2.20 compared with 2.21, p = 0.79) nor the radius ratio (0.75 compared with 0.73, p = 0.53) differed significantly between these two groups.
The prevalence of ACL injuries has led investigators to investigate the factors that are associated with these injuries and factors that could explain the predisposition of female athletes toward this injury. We chose to focus on the lateral tibiofemoral geometry because translational and rotational perturbations across the lateral compartment lead to or result from ACL injury. Previous studies demonstrated that the femur has a nearly spherical shape at its point of contact with the tibia31. The morphology of the lateral tibial plateau changes substantially from its medial aspect at the tibial spine (where it is convex) to the lateral edge of the joint (where it flattens)29. The length and shape of the lateral aspect of the plateau at the midsagittal plane vary among individuals, but in most individuals the plateau is convex at this point. We hypothesized that individuals with an ACL injury would demonstrate greater curvature of the articular surfaces than individuals without such an injury and that male-female differences in lateral tibiofemoral geometry could help to explain the female predisposition toward ACL injury.
We developed a novel measurement protocol that could be carried out with use of readily available software and generated an easily reproducible set of measurements for evaluating the lateral tibiofemoral geometry. The curvatures measured by our three independent observers demonstrated “almost perfect” agreement (see Appendix).
A power analysis was performed to determine the size of the cohort that would be needed to identify meaningful differences in the measurements. A homogeneous population of individuals was identified through retrospective review of the charts of a series of consecutive patients. Strict inclusion criteria minimized bias due to differences in activity level between the ACL-injured and uninjured groups. However, small differences in activity level with regard to age and sex did remain. For instance, the female cohort had higher Tegner scores than the male cohort; only 65% of female patients had a Tegner score of 6 or 7 compared with 83% of male patients, whereas 28% of female patients had a Tegner score of 9 compared with only 14% of male patients. These differences reflect the demographic distribution of patients seen at our clinic, which treats a large population of students and National Collegiate Athletic Association (NCAA) Division-I athletes.
When injured and uninjured patients were combined, the mean femoral and tibial anteroposterior lengths and radii of curvature were smaller in female patients than in male patients because of male-female differences in stature. However, the tibial plateau anteroposterior length relative to the femoral anteroposterior length demonstrated significant male-female differences, with the female tibial plateau appearing to be relatively shorter and more convex. This held true even if ACL-injured patients were dropped from the analysis and if size differences were controlled for with use of length and radius ratios.
When the sexes were combined, ACL-injured patients had a tibial articular surface that was significantly shorter relative to that of the femur and had a steeper convexity (smaller radius of curvature) compared with uninjured patients. The distal femoral radius of curvature also differed significantly.
A comparison of ACL-injured and uninjured male patients (Table V) revealed that the tibial and femoral surfaces were more convex and the anteroposterior length of the tibial plateau was smaller in the injury group. Since these differences could have been size-related (i.e., smaller men may have a smaller ACL that is more prone to tearing), we also analyzed the length and radius ratios and observed that the differences remained significant. In contrast, a comparison of injured and uninjured female patients (Table IV) revealed no differences with regard to the femoral or tibial anteroposterior length, femoral or tibial radius of curvature, or length or radius ratios.
Interestingly, the relative length and curvature ratios of ACL-injured male patients did not differ significantly from those of either ACL-injured female patients or uninjured female patients. One possible interpretation of this finding is that an “at-risk” lateral tibiofemoral geometry exists and is shared between some men and most women.
Anthropometric sex differences that represent potential risk factors for ACL injury have been widely studied. It is likely that non-contact ACL injuries are caused by a combination of variables working in concert. The risk factors that have been studied include knee-specific variables such as the narrowness of the intercondylar notch15-18,34,35 and the size of the ACL19 as well as other variables such as the length and height of the tibia or thigh21, ligamentous hyperlaxity20,21, body mass index21, and the Q angle22.
The effect of tibial slope on knee stability and ACL injury remains controversial23,25-27,36. Meister et al. noted no significant difference in the caudal slope of the lateral aspect of the tibia between patients who had sustained a non-contact ACL injury and an age-matched cohort of patients with patellofemoral pain syndrome, but those authors did not stratify patients according to sex25. Brandon et al. found that there was an association between increased posterior inferior lateral tibial slope (PITS), ACL injury, and pivot-shift grade in patients with ACL insufficiency23. Male-female differences in these parameters were not observed. In a similar comparison study, Stijak et al.26 noted that ACL injury was associated with a greater lateral tibial slope but with a smaller medial tibial slope, and therefore advocated measurement of both the medial and the lateral slope. Todd et al.27 noted that ACL-injured patients had a greater tibial slope compared with control subjects; when stratified according to sex, the difference was only significant in female subjects. The abovementioned studies indicate that tibial slope may be a risk factor for ACL injury, but this parameter does not account for the female predisposition toward ACL injury.
Recent investigations by Hashemi et al. identified variability in the geometry of the tibial plateau in the sagittal and coronal planes24,28. Female subjects had greater mean medial and lateral tibial slopes, and male subjects had a greater coronal tibial slope24. In a follow-up study, the authors noted that ACL-injured female and male subjects had a greater lateral tibial slope and a shallower medial tibial depth compared with uninjured controls. ACL-injured male subjects also had a significantly greater medial tibial slope compared with controls. Logistic regression analysis indicated that medial tibial depth and lateral tibial slope were risk factors for ACL injury in both sexes, whereas medial tibial slope was only a risk factor in men28.
Musahl et al. investigated coronal-plane geometry and found that a narrow lateral aspect of the tibial plateau correlated with a higher-grade pivot shift in women but not in men30.
Siebold et al. characterized differences in the radii of curvature of the medial and lateral femoral condyles in ACL-injured knees and in the uninjured contralateral knees. The sagittal radius of curvature was similar between the medial and lateral condyles, but both the coronal and the axial curvature differed greatly. No differences were identified between the ACL-injured knees and the contralateral normal knees, and sex differences were not studied31.
Although the convex shape of the lateral aspect of the tibial plateau has long been recognized, few studies have attempted to correlate the degree of convexity with the risk of ACL rupture. Matsumoto studied the mechanism of the pivot shift on ACL-intact and ACL-sectioned cadaveric knees with use of biplanar photography9. Seven (35%) of twenty knees demonstrated a pivot shift after sectioning of the ACL. The remaining thirteen knees demonstrated abnormal internal rotation but no pivot shift. Matsumoto qualitatively noted that a flatter surface of the lateral aspect of the tibial plateau was associated with clinical stability despite ACL deficiency. Kujala et al.29 studied twenty subjects with a chronic ACL tear and determined whether they had a clinical history of giving-way or demonstrated a pivot shift during physical examination. MRI measurements of the height of the convexity of the lateral tibial plateau correlated with clinical signs and symptoms of instability29.
To our knowledge, our study is the first to quantitatively and qualitatively identify differences in the geometry of the lateral tibiofemoral articular surface that may help to explain the risk of ACL injury and the female predisposition toward this injury. Our results suggested that there exists an “at-risk” geometric phenotype of the lateral tibiofemoral articular surface, characterized by a shorter and more convex tibial plateau relative to the femur and by a smaller and more convex distal femoral surface, which may render the knee more susceptible to ACL injury. This geometry was nearly universal among female patients in our study population and was present in male patients who had sustained an ACL injury, but it was not characteristically observed in uninjured male patients. The data corroborate the findings of Salmon et al.37, who reported that female subjects were not significantly more likely than male subjects to sustain an ipsilateral ACL reinjury after ACL reconstruction. Shelbourne similarly noted that female subjects were no more likely than male subjects to sustain an ACL reinjury after reconstruction, although both male and female subjects who sustained an ACL injury were more likely than the general population to sustain a subsequent ACL injury38.
Our study has several limitations. First, the design of the study was retrospective, although it would be difficult and costly to image and follow uninjured subjects prospectively in anticipation of ACL injuries. Second, we attempted to standardize our ACL-injured and uninjured groups to eliminate bias due to differences in activity level but we could not eliminate this difference entirely. Third, our measurement protocol employed a single sagittal-plane image—a two-dimensional representation of the complex three-dimensional joint morphology. However, we believe that using the apex of the tibiofemoral region of contact was a reasonable approximation, and a similar method was employed in other published MRI anatomic studies by Hashemi et al.24,28 and Musahl et al.30. Fourth, sequelae of ACL injury (bone bruising) were visible in sixty-one (35%) of the 173 MRIs studied; thus, although the observers were completely blinded to age and sex in all cases, they could not be uniformly blinded to injury status. However, given that our findings were not consistent across sex and injury groups and given that the ICC indicated substantial to near perfect agreement between observers, this was likely more a theoretical than an actual bias.
Finally, simultaneous sex-specific variations in intercondylar notch width, ACL size, hormonal effects, and articular width and curvature may complicate efforts to determine the roles of the individual geometric parameters, especially those that are not the most important contributory factors. Weighing and stratifying the importance of these individual variables will require conducting multivariate studies in robust study populations that include large female cohorts.
In summary, we have described an easily reproducible and consistent method for assessing the radii of curvature of the lateral aspect of the tibiofemoral joint. To our knowledge, this is the first study to quantitatively analyze the radial geometry of the articular surfaces of the lateral compartment of the knee joint and correlate the resulting objective findings with the risk of ACL injury, including the female predisposition toward this injury. Further clarification of the factors that lead to ACL injury may eventually allow surgeons to identify at-risk individuals.
Our data indicated that ACL-injured male patients and nearly all female patients shared a common lateral tibiofemoral geometry characterized by a relatively short and steeply convex tibial articular surface and increased convexity of the femoral surface. Among a multitude of factors that are likely to be involved in the development of ACL injuries, it may be geometry as much as female sex that predisposes individuals toward this injury. Further studies into the three-dimensional anatomy of the lateral aspect of the tibiofemoral joint as well as the biomechanical consequences of a short and/or highly convex lateral bearing surface are warranted.
Tables providing demographic information on the study groups and ICC values for the geometric parameters as well as figures illustrating the determination of the geometric parameters are available with the online version of this article as a data supplement at jbjs.org.
Note: The authors would like to thank Roger V. Larson, MD, for his consideration, guidance, and generosity in the preparation of this manuscript.
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