Changes in Cross-sectional Area and Signal Intensity of Healing Anterior Cruciate Ligaments and Grafts in the First 2 Years After Surgery

Abstract

Background: The quality of a repaired anterior cruciate ligament (ACL) or reconstructed graft is typically quantified in clinical studies by evaluating knee, lower extremity, or patient performance. However, magnetic resonance imaging of the healing ACL or graft may provide a more direct measure of tissue quality (ie, signal intensity) and quantity (ie, cross-sectional area).

Hypotheses: (1) Average cross-sectional area or signal intensity of a healing ACL after bridge-enhanced ACL repair (BEAR) or a hamstring autograft (ACL reconstruction) will change postoperatively from 3 to 24 months. (2) The average cross-sectional area and signal intensity of the healing ligament or graft will correlate with anatomic features of the knee associated with ACL injury.

Study design: Cohort study; Level of evidence, 2.

Methods: Patients with a complete midsubstance ACL tear who were treated with either BEAR (n = 10) or ACL reconstruction (n = 10) underwent magnetic resonance imaging at 3, 6, 12, and 24 months after surgery. Images were analyzed to determine the average cross-sectional area and signal intensity of the ACL or graft at each time point. ACL orientation, stump length, and bony anatomy were also assessed.

Results: Mean cross-sectional area of the grafts was 48% to 98% larger than the contralateral intact ACLs at all time points (P < .01). The BEAR ACLs were 23% to 28% greater in cross-sectional area than the contralateral intact ACLs at 3 and 6 months (P < .02) but similar at 12 and 24 months. The BEAR ACLs were similar in sagittal orientation to the contralateral ACLs, while the grafts were 6.5° more vertical (P = .005). For the BEAR ACLs, a bigger notch correlated with a bigger cross-sectional area, while a shorter ACL femoral stump, steeper lateral tibial slope, and shallower medial tibial depth were associated with higher signal intensity (R2 > .40, P < .05). Performance of notchplasty resulted in an increased ACL cross-sectional area after the BEAR procedure (P = .007). No anatomic features were correlated with ACL graft size or signal intensity.

Conclusion: Hamstring autografts were larger in cross-sectional area and more vertically oriented than the native ACLs at 24 months after surgery. BEAR ACLs had a cross-sectional area, signal intensity, and sagittal orientation similar to the contralateral ACLs at 24 months. The early signal intensity and cross-sectional area of the repaired ACL may be affected by specific anatomic features, including lateral tibial slope and notch width-observations that deserve further study in a larger cohort of patients.

Registration: NCT02292004 (ClinicalTrials.gov identifier).

Keywords: ACL; BEAR; MRI; bridge-enhanced ACL repair; reconstruction; repair; signal intensity; size.

Figures

Figure 1.

Flow diagram of patient recruitment and follow up. *The total number of patients not meeting inclusion criteria totals to greater than 214, as some patients met more than 1 exclusion criterion. #Not all patients completed every component of the follow-up assessments. Sample sizes with data available for analysis for each component are shown in the figures and tables.

Figure 2.

Stepwise demonstration of the “Bridge-Enhanced ACL Repair” technique using the scaffold. In this technique, the torn ACL tissue is preserved (A). A whip stitch of #2 absorbable suture (purple suture) is placed into the tibial stump of the ACL. Small tunnels (4 mm) are drilled in the femur and tibia and a suture cinch comprised of a cortical button with two #2 non-absorbable sutures (green sutures) and the #2 absorbable ACL sutures are passed through the femoral tunnel and the button engaged on the proximal femoral cortex. The non-absorbable sutures are threaded through the scaffold, tibial tunnel and secured in place with a second extracortical button. The scaffold is then saturated with 10 mL of the patient’s blood (B), and the tibial stump pulled up into the saturated scaffold (C). The ends of the torn ACL then grow into the scaffold and the ligament reunites (D). Used with permission from Murray et al, OJSM 2016.

Figure 3.

Time-dependent changes in ACL/graft (A) cross-sectional area and (B) signal intensity. (C) Sample size for each treatment group at every time point. Intact ACL values are from the contralateral intact knee imaged at 24 months. Mean±standard deviation. *p<.02 for comparisons to the contralateral intact acl. repaired acl cross-sectional area at months compared smaller reconstructed graft and months.># Standard deviations are smaller than the symbol (± 2.2 mm2). BEAR: Bridge-Enhanced ACL Repair; ACLR: ACL reconstruction.

Figure 4:

Associations between BEAR ACL signal intensity at 24 months with (A) femoral stump length (normalized to ACL length), (B) lateral tibial slope, and (C) medial tibial depth. N= 9. Dotted lines represent the 95% confidence intervals. Horizontal line represents the average signal intensity of the contralateral intact ACL.

Figure 5:

Associations between ACL cross-sectional area of the BEAR group at 24 months and pre-operatively notch width (A) and post-operative notch width (B), and (C) notchplasty. N= 9. Dotted lines represent the 95% confidence intervals. Horizontal line represents the average cross-sectional area of the contralateral intact ACL.