Morphologic Factors Associated With Secondary Anterior Cruciate Ligament Injury After Primary Anterior Cruciate Ligament Reconstruction

Abstract

Background:

Bone morphology is a well-recognized risk factor for primary and secondary anterior cruciate ligament (ACL) injury.

Purpose:

To evaluate lower-extremity bone morphology using computed tomography (CT) in patients after primary ACL reconstruction and investigate the association between the risk of ipsilateral graft rupture and contralateral ACL injury.

Study Design:

Case-control study; Level of evidence, 3.

Methods:

This study included 224 consecutive patients who underwent primary ACL reconstruction at our hospital. Postoperative CT scans obtained within 1 week were used to determine the following morphological parameters: femoral neck anteversion, medial and lateral posterior tibial slope (PTS), femoral notch width, notch width index, medial tibial spinal height (MTSH), lateral tibial spinal height, and tibial torsion. These bony morphological parameters were compared between patients with and without graft rupture and between those with and without contralateral injury. Multivariate logistic regression was performed to identify independent risk factors for graft failure and contralateral ACL injury.

Results:

Graft rupture occurred in 16 patients (7.1%), and contralateral ACL injuries occurred in 22 patients (9.8%). Patients with graft rupture had smaller MTSH than those without graft rupture (P < .01), and it was significantly associated with graft rupture risk (odds ratio [OR], 0.59). Patients with contralateral injuries had a greater tibial torsion (P = .01), which was associated with contralateral injury risk (OR, 1.08).

Conclusion:

In patients who underwent primary ACL reconstruction, a smaller MTSH was associated with a higher risk of graft rupture, whereas a greater tibial torsion was associated with a higher risk of contralateral ACL injury.

Keywords

anterior cruciate ligament (ACL) injury ACL reconstruction reinjury graft failure contralateral injury morphology

Anterior cruciate ligament (ACL) injury is a common and serious injury in athletes, with a substantial risk of secondary ACL injury after surgical treatment. Secondary ACL injury after primary ACL reconstruction is reported to occur at a high rate, with ipsilateral graft failure in 7.7% to 12.0% and contralateral ACL injury in up to 6.9% to 11.8% of patients.^19,20,25 Although ACL injuries can result from isolated trauma, multiple intrinsic factors, such as sex, anatomical morphology, neuromuscular control, genetic predisposition, and family history, may increase susceptibility.^4,5 After primary ACL reconstruction, the mechanisms underlying reinjury are even more complex, involving traumatic events, surgical technique–related variables, and biological factors.^26,28,32,33 Risk factors for the initial ACL injury have also been identified as strong predictors of graft rupture or contralateral injury.^32,33

Bone morphology is a well-recognized nonmodifiable risk factor for primary or secondary ACL injury. The most frequently reported parameters include the posterior tibial slope (PTS),^3,30 femoral intercondylar notch width,³¹ and frontal knee alignment.²⁴ Previous investigations have assessed only some bony morphological parameters that have been identified as potential risk factors, and there has been substantial heterogeneity in imaging modalities and participant characteristics across studies. Considering the multifactorial nature of secondary ACL injuries, a comprehensive assessment of multiple bone morphological parameters may improve risk stratification. This study aimed to evaluate lower extremity bone morphology using computed tomography (CT) in patients after primary ACL reconstruction and investigate its association with the risk of ipsilateral graft rupture and contralateral ACL injury. We hypothesized that specific bone morphological features could predict the risk of secondary ACL injury.

Methods

Patients

After institutional review board approval, a retrospective analysis of prospectively collected data was conducted. The opt-out method was used to obtain consent for this study. Between 2017 and 2019, 266 consecutive patients underwent primary ACL reconstruction at our hospital. The exclusion criteria were as follows: (1) multiple knee ligament injury, except for those with injury to the medial collateral ligament (n = 4); (2) previous ACL injury to the contralateral knee (n = 4); (3) knee osteoarthritis (Kellgren-Lawrence (KL) grade ≥ 2) (n = 2); (4) combined fractures, except for those with Segond fractures (n = 1); (5) combined knee surgery with medial patellofemoral ligament reconstruction (n = 1); and (6) lack of image data or loss to follow-up (n = 30). In total, 224 patients met the above criteria and were followed for a minimum of 24 months postoperatively (Figure 1).

Figure 1.

Patient flowchart. ACL; anterior cruciate ligament, KL; Kellgren-Lawrence, MPFL; medial patellofemoral ligament.

Surgical Procedure and Postoperative Rehabilitation

Anatomic ACL reconstruction was performed with a single-bundle technique using an autograft rectangular bone–patellar tendon–bone (BTB) graft (n = 21) or a double-bundle technique using an autograft hamstring tendon (HT) graft (n = 203).^22,23 In ACL reconstruction, femoral tunnels were created behind the lateral intercondylar ridge, and tunnels were created in the tibial ACL footprint. Concomitant meniscal injuries were treated appropriately when necessary.¹⁴ The same protocol used after ACL reconstruction was adopted for postoperative rehabilitation. The patients began crutch-assisted partial-body weightbearing ambulation and isometric muscle-strengthening exercises the day after the surgery. The knee joint was maintained in extension with a knee brace for 1 week after surgery. Closed kinetic chain exercises were commenced after 2 weeks and full weightbearing gait was permitted between 2 and 3 weeks. Range of motion exercises were started after 2 weeks and increased gradually to achieve a full range of motion within 6 weeks. Running, open kinetic chain exercises, and jump-landing training were permitted after 3 months, sports-specific training was allowed after 5 to 6 months, and return to sports activities was permitted after 6 to 9 months.

Data Collection

Demographic data, such as age at the time of primary ACL reconstruction, sex, height, body weight, body mass index (BMI), and graft type were obtained from medical records. In addition, preinjury sports participation was recorded, and the time to return to sport after ACL reconstruction was evaluated in those who participated in sports prior to injury. Return to sport was defined as resumption of regular training or competitive play.

Patients were followed up through in-person clinical evaluations for a minimum of 24 months postoperatively. During this period, follow-up assessments were conducted through scheduled outpatient visits. Thereafter, in cases of suspected graft rupture or contralateral ACL injury, patients were directly evaluated through assessment of the traumatic mechanism, physical examination, and magnetic resonance imaging (MRI). In all such cases, the diagnosis of graft rupture or contralateral injury was confirmed by arthroscopic findings at the time of revision or reconstructive surgery. The final follow-up interval was defined differently depending on patient status. For patients without secondary injury, it was defined as the duration from surgery to the last outpatient follow-up. For patients who experienced graft rupture or contralateral injury, it was defined as the duration from surgery to the time of secondary ACL injury.

To obtain lower limb morphological parameters, we used CT scans that had been obtained within 1 week postoperatively as part of routine clinical care to verify the bone tunnel position after ACL reconstruction. The femoral neck anteversion was measured using the Jarrett method.¹³ A line was drawn on a single axial oblique image that ran from the center of the femoral head through the center of the femoral neck. The angle was measured using a tangential line through the distal femoral condyles (Figure 2). The medial and lateral PTS (MPTS and LPTS) were measured using the Hudek method. The tibial axis was defined as the line connecting the midpoints of the outer cortical diameters at 2 circles, and the slopes were measured at the medial and lateral centers of the articular surface (Figure 3).¹¹ The intercondylar notch width at the most inferior point of the notch and the notch width index were defined as a ratio between the widths of the intercondylar notch and the distal femur (when measured at the level of the popliteal groove) in the coronal plane (Figure 4).¹⁷ The height of the medial and lateral tibial spines (MTSH and LTSH) was measured in the coronal plane using the technique described by Cavaignac et al⁶ (Figure 5). To account for interindividual differences in tibial size, normalized values of MTSH and LTSH were calculated by dividing each by tibial width (normalized MTSH and LTSH). Tibial torsion was measured as the angle between a line along the proximal posterior tibial plateau and a line connecting the centers of the medial and lateral malleoli (Figure 6).²¹

Figure 2.

Femoral neck anteversion. Femoral neck anteversion was defined as the angle between a line drawn from the center of the femoral head through the center of the femoral neck on a single axial oblique image (yellow line) and a tangential line through the distal femoral condyles (white line).

Figure 3.

Medial and lateral posterior tibial slopes. The tibial axis was defined as the line connecting the midpoints of the outer cortical diameters at 2 circles, and the slopes were measured at the (A) medial and (B) lateral centers of the articular surface.

Figure 4.

Intercondylar notch width index. The notch width index was defined as a ratio between the widths of the intercondylar notch (yellow line) and the distal femur in the coronal plane (white dashed line).

Figure 5.

Height of the medial and lateral tibial spines. The height of the medial (yellow line) and lateral tibial spines (blue line) were measured as the perpendicular distance from the peak of each spine and a line connecting the peak points on the medial and lateral aspects of the plateau (white dashed line).

Figure 6.

Tibial torsion. Tibial torsion was measured as the angle between a line along the proximal posterior tibial plateau (white line) and a line connecting the centers of the medial and lateral malleoli (yellow line).

All measurements were performed by an orthopaedic resident (L.C.) who was blinded to the patients’ histories and reinjury status. To determine intraobserver reliability, intraclass correlation coefficients (ICCs) were calculated from all measurements of 20 randomly selected patients performed twice ≥2 weeks apart. The intraobserver ICCs ranged from 0.90 to 0.96. To assess interobserver reliability, an additional board-certified orthopaedic surgeon (Y.K.) independently performed the same measurements. The interobserver ICCs ranged from 0.78 to 0.88.

Statistical Analysis

Demographic and morphologic data are presented as mean ± SD. Group comparisons were performed using the Wilcoxon rank-sum test for continuous variables and the chi-square test or Fisher exact test for categorical variables. Multivariate logistic regression was performed to identify independent risk factors for graft failure and contralateral ACL injury. Morphological variables with P < .05 in univariate analysis were included in the model. All statistical analyses were performed using SPSS Version 29.0 (SPSS Inc). A P < .05 was considered statistically significant.

Results

Of the 224 patients, 16 (7.1%) experienced ipsilateral graft rupture, whereas 22 (9.8%) sustained a contralateral ACL injury. Table 1 summarizes the demographic data of patients with and without graft rupture. There were no significant differences in height, weight, BMI, graft type, or time from ACL reconstruction to return to sports between the groups. Patients with graft failure were significantly younger than those without graft rupture (P < .001).

Table 1

Demographic Data for Graft Rupture ^a

	Graft Rupture(n = 16)	No Graft Rupture(n = 208)	P
Age, y	17.3 ± 5.9	25.8 ± 12.6	<.001
Sex, female:male	9:7	114:94	.91
Height, cm	164.1 ± 6.3	164.9 ± 9.0	.73
Weight, kg	61.7 ± 10.5	66.4 ± 14.2	.20
BMI, kg/m²	22.8 ± 2.6	24.3 ± 3.9	.14
Graft type, HT:BTB	14:2	189:19	.67
Rate of return to sports, %	93.4 (15/16)	82.8 (144/174)^b	.48
Time from ACL reconstruction to return to sports, mo	8.8 ± 2.4	10.7 ± 3.7	.15
Final follow-up interval, mo, median (mean ± SD)	18 (19.1 ± 11.5)	24 (28.6 ± 13.3)	.007

Data are presented as n or mean ± SD unless otherwise indicated. Bold P values indicate statistically significant differences. ACL, anterior cruciate ligament; BMI, body mass index; BTB, bone–patellar tendon–bone; HT, hamstring tendon.

Among patients who participated in sports before injury (n = 174).

Association Between Morphologic Parameters and Graft Rupture

Patients with graft rupture had smaller MTSH than those without graft rupture (P < .01) (Table 2). Similarly, normalized MTSH was also significantly lower in patients with graft rupture (P = .04). In the multivariate analysis, MTSH was inversely associated with the risk of graft rupture (odds ratio [OR], 0.59) (Table 3), indicating that greater MTSH was associated with a lower risk of graft rupture. No other morphological variables differed significantly between the groups.

Table 2

Morphologic Parameters in Graft Rupture ^a

	Graft Rupture	No Graft Rupture	P
	(n = 16)	(n = 208)	P
Femoral neck anteversion, deg	13.31 ± 5.70	10.80 ± 6.52	.14
Medial PTS, deg	5.18 ± 1.60	5.09 ± 2.08	.43
Lateral PTS, deg	5.31 ± 1.16	5.53 ± 2.20	.38
Notch width, mm	18.46 ± 1.77	18.32 ± 2.45	.82
Bicondylar width, mm	70.21 ± 4.86	71.25 ± 6.63	.44
Notch width index	0.26 ± 0.02	0.26 ± 0.02	.51
MTSH, mm	7.28 ± 1.86	8.43 ± 1.61	<.01
LTSH, mm	6.64 ± 1.79	7.35 ± 1.85	.15
Normalized MTSH	0.10 ± 0.02	0.12 ± 0.02	.04
Normalized LTSH	0.10 ± 0.03	0.10 ± 0.02	.28
Tibial torsion, deg	33.32 ± 9.71	34.72 ± 8.61	.53

Data are reported as mean ± SD. Bold P values indicate statistically significant differences. LTSH, lateral tibial spinal height; MTSH, medial tibial spinal height; PTS, posterior tibial slope.

Table 3

Risk Factors for Graft Rupture ^a

	B	P	Odds Ratio	95% CI
Femoral neck anteversion	0.06	.14	1.06	0.98-1.15
Medial PTS	–0.04	.81	0.96	0.71-1.30
Lateral PTS	–0.04	.77	0.96	0.72-1.28
Notch width	0.25	.13	1.28	0.93-1.78
Notch width index	–3.69	.84	0.02	<0.01-3.8 × 10¹³
MTSH	–0.53	.01	0.59	0.39-0.88
LTSH	–0.19	.33	0.83	0.56-1.21
Tibial torsion	–0.04	.24	0.96	0.90-1.03

LTSH, lateral tibial spinal height; MTSH, medial tibial spinal height; PTS, posterior tibial slope. Bold P values indicate statistically significant differences.

Association Between Morphologic Parameters and Contralateral ACL Injury

Table 4 presents the morphologic parameters for patients with and without contralateral ACL injury. Patients with contralateral ACL injury were significantly younger (P < .01) and more likely to be female (P = .03) than those without injury. Table 5 shows the morphologic parameters in the patients with contralateral injury. Patients with contralateral injury had greater tibial torsion (P = .01) compared to those without contralateral injury. Patients with contralateral ACL injury did not show a significant difference in return-to-sport rates compared with those without contralateral injury; however, they returned to sport significantly earlier after surgery (P < .001). In multivariable models, large tibial torsion was associated with contralateral injury (OR, 1.08) (Table 6). No significant differences were found in the other morphological parameters.

Table 4

Demographic Data for Contralateral ACL Injury ^a

	Contralateral Injury(n = 22)	No Contralateral Injury(n = 202)	P
Age, y	19.0 ± 8.6	25.8 ± 12.6	<.01
Sex, female:male	17:5	106:96	.03
Height, cm	162.1 ± 9.0	165.2 ± 8.9	.18
Weight, kg	61.3 ± 10.8	66.6 ± 14.2	.09
BMI, kg/m²	23.2 ± 2.4	24.3 ± 4.0	.08
Graft type, HT:BTB	21:1	182:20	.42
Rate of return to sports, %	95.5 (21/22)	^b83.3 (140/168)	.21
Time from ACL reconstruction to return to sports, mo,	9.0 ± 1.2	10.9 ± 3.8	<.001
Final follow-up interval, mo median (mean ± SD)	24 (29.2 ± 1.2)	24 (29.2 ± 14.2)	.72

Among patients who participated in sports before injury (n = 168).

Table 5

Morphologic Parameters in Patients With Contralateral ACL Injury ^a

	Contralateral Injury	No Contralateral Injury	P
	(n = 22)	(n = 202)	P
Femoral neck anteversion, deg	9.97 ± 6.32	11.10 ± 6.51	.44
Medial PTS, deg	5.33 ± 2.11	5.32 ± 1.96	.77
Lateral PTS, deg	4.86 ± 1.16	5.57 ± 2.10	.18
Notch width, mm	18.07 ± 1.72	18.36 ± 2.47	.60
Bicondylar width, mm	70.80 ± 6.09	71.30 ± 6.55	.39
Notch width index	0.26 ± 0.02	0.26 ± 0.02	.91
MTSH, mm	7.82 ± 1.47	8.49 ± 1.69	.11
LTSH, mm	6.70 ± 1.70	7.35 ± 1.87	.17
Normal MTSH	0.11 ± 0.02	0.12 ± 0.02	.09
Normal LTSH	0.10 ± 0.02	0.10 ± 0.02	.19
Tibial torsion, deg	38.90 ± 7.81	34.15 ± 8.66	.01

Data are reported as mean ± SD. Bold P values indicate statistically significant differences. ACL, anterior cruciate ligament; LTSH, lateral tibial spinal height; MTSH, medial tibial spinal height; PTS, posterior tibial slope.

Table 6

Risk Factors for Contralateral ACL Injury ^a

	B	P	Odds Ratio	95% CI
Femoral neck anteversion	–0.06	.17	0.94	0.86-1.03
Medial PTS	0.14	.27	1.15	0.90-1.46
Lateral PTS	–0.21	.11	0.81	0.62-1.05
Notch width	0.02	.90	1.02	0.80-1.30
Notch width index	1.03	.94	2.81	<0.01-2.5 × 10¹²
MTSH	–0.15	.40	0.86	0.60-1.22
LTSH	0.00	≥.99	1.00	0.72-1.38
Tibial torsion	0.08	.01	1.08	1.02-1.15

LTSH, lateral tibial spinal height; MTSH, medial tibial spinal height; PTS, posterior tibial slope.

Discussion

The present study revealed that a smaller MTSH was associated with a higher risk of ipsilateral ACL graft rupture and that a larger tibial torsion was associated with a higher risk of contralateral ACL injury. The femoral neck anteversion, MPTS, LPTS, notch width, and notch width index were not significant risk factors.

Several studies have compared the tibial spinal morphology of patients who underwent ACL reconstruction with that of control participants without a knee injury history. The main finding of their study was that the total tibial spinal volume in the ACL-injured group was smaller than that in the ACL-intact group.^12,29 Levins et al¹⁵ investigated geometric risk factors in 11 patients after ACL reconstruction with graft rupture and 44 patients without graft rupture and found that female patients with reduced MTSH and medial tibial spine volume had a higher risk of ACL graft rupture. However, there were no common risk factors for ACL graft rupture and first-time ACL injuries in male patients. The authors also noted that small sample size limited the statistical significance of their findings. In the present study, the MTSH was significantly smaller in the group that experienced graft rupture and was identified as a risk factor for graft rupture. Age-related changes in knee morphology may also influence this finding. Previous studies have demonstrated that tibial spinal height significantly increases with age during skeletal growth and maturation.¹⁰ Therefore, it is possible that age-related differences in tibial spinal morphology may have influenced this association. However, due to the limited number of patients with secondary ACL injury, further adjustment for age and other potential confounders was not feasible in the current analysis. Further studies with larger, multicenter cohorts and longer follow-up periods are needed to confirm whether a smaller MTSH truly represents a risk factor for graft rupture requires and to further clarify the role of tibial torsion in contralateral ACL injury.

Numerous studies have demonstrated that altered biomechanics in the transverse plane play a central role in the mechanism of ACL injury and that abnormal torsion modifies rotational stresses, increasing the risk of ACL injury.^1,2,27 Alpay et al¹ compared femoral anteversion using MRI between ACL-deficient and ACL-intact groups and found that femoral anteversion was higher in the ACL-deficient group than in the control group. In the present study, femoral anteversion did not differ among the ACL-injured, graft rupture, and contralateral injury groups and was not associated with the risk of secondary ACL injury. This discrepancy may be partly explained by the fact that many previous studies compared ACL-injured patients with ACL-intact control groups. Wang et al²⁷ compared distal femoral torsion, posterior femoral condylar torsion, and proximal tibial torsion on MRI and compared the ACL-deficient and ACL-intact groups. In patients with ACL injury, the measurements of distal femoral torsion and posterior femoral condylar torsion were significantly higher than those in the control group. However, proximal tibial torsion did not differ between the 2 groups. In this study, greater tibial torsion was observed in patients who sustained a contralateral ACL injury. No study has demonstrated an association between tibial torsion and ACL injury or reinjury. In previous studies, MRI was usually limited to the knee or hip joint, and simultaneous imaging from the hip to the ankle has rarely been performed. Therefore, a comprehensive evaluation using images encompassing the entire lower limb, as in the present study, was not possible.

Biomechanical studies have shown that increased PTS may affect anteroposterior knee laxity and tibial shear forces, ultimately increasing the risk of graft failure after ACL reconstruction.¹⁸ Systematic review and meta-analysis showed that MPTS and LPTS were associated with a higher risk of ACL injury and graft rupture.^16,31 In the present study, neither MPTS nor LPTS was identified as a risk factor for graft rupture or contralateral ACL injury. Some studies have also reported no association between MPTS and ACL graft failure or functional outcomes.^7,8,9 Differences between studies may be related to several factors. First, in the present study, the second ACL injuries were traumatic rather than graft failures, which may account for the absence of a significant difference. Second, variations in patient demographics such as race and age may have influenced the results. Finally, most previous studies used plain radiographs or MRI for PTS measurement, which could have contributed to methodological discrepancies.

Second, ACL injury is multifactorial, and static morphologic parameters alone cannot fully explain this complex clinical problem. Although earlier return to sport may contribute to reinjury risk, the return-to-sport timing in our cohort was within the range commonly reported after primary ACL reconstruction. Therefore, our findings should be interpreted in the context of multifactorial risk factors for ACL reinjury, rather than as a recommendation for routine CT screening in all patients. Instead, CT-based morphologic features may contribute to risk stratification and prognostic assessment in selected patients. These findings may help identify high-risk patients and guide individualized rehabilitation and return-to-sport strategies.

Limitations

The present study has some limitations. First, this was a single-center retrospective study with a minimum 2-year follow-up. Reports indicate that the incidence of a second ACL injury increases with a longer follow-up period. Second, although return-to-sport rate and timing were evaluated, detailed information on postreconstruction activity exposure, such as training intensity and playing time, was not collected. Therefore, the influence of activity level on secondary ACL injury could not be fully assessed. Third, coronal alignment parameters, such as the femorotibial angle, were not evaluated in this study. Accurate assessment of coronal alignment was not feasible using the current CT imaging protocol because of limitations in slice orientation and the lack of full-length weightbearing images. Finally, anatomical and surgical factors (graft healing and location of the femoral or tibial bone tunnel), movement patterns (kinematics or kinetics obtained from motion analysis), and functional data were not included in the analysis.

Conclusion

In patients who underwent primary ACL reconstruction, a smaller MTSH was associated with an increased risk of graft rupture, whereas a greater tibial torsion was associated with a higher risk of contralateral ACL injury.

Footnotes

Final revision submitted April 17, 2026; accepted April 18, 2026.

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution.

Ethical approval for this study was obtained from Hirosaki University Graduate School of Medicine.

ORCID iDs

Yuka Kimura

Kyota Ishibashi

Eiji Sasaki

Eiichi Tsuda

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