Summary
This study highlights that the critical postoperative period between 3 and 6 months after ACL reconstruction is characterized by significant biological changes, where MRI assessments can guide rehabilitation to optimize recovery and reduce re-injury risks.
Abstract
Background
Rehabilitation following anterior cruciate ligament reconstruction (ACLR) should be individualized, with a focus on criterion-based assessments of functional capabilities. MRI evaluations of the knee joint are not routinely conducted during recovery. This study aimed to identify structural changes in the postoperative knee and determine the timing of these changes to guide rehabilitation strategies and optimize recovery outcomes as athletes progress toward returning to sport.
Methods
Seventy-five patients (42 males, 33 females) who underwent reconstruction with autologous quadriceps tendon grafts were reviewed via 3-T MRI at 3, 6, 9, 12, 18, and 24 months post-ACLR. A control group comprised 84 healthy adults with intact ACLs. All surgeries were performed by the same orthopedic surgeon, and the same physiotherapy team conducted rehabilitation. Signal intensity (SI) was measured manually in three ROIs for the ACL and one ROI for the PCL. The ACL/PCL ratio (APR) was calculated and compared over time (3 to 24 months) and to native ACL signal intensity assessed through the same method. Bone marrow edema (BME) was evaluated in coronal STIR sections by measuring signal intensity in five areas: lateral and medial femoral and tibial condyles.
Clinical examinations assessed knee laxity performed with the Rolimeter arthrometer, range of motion, localized pain, and effusion presence.
Results
The most significant changes in the knee occurred between 3 and 6 months postoperatively. Three months after surgery, the APR was 2.66 (1.89-3.86). During this period, SI increased, reaching its peak at 6 months with a value of 3.69 (2.82-4.64). At this peak, remodeling achieved approximately 56% of the native value (2.02 ± 0.68, p < 0.01). After reaching its highest point, SI gradually declined until 24 months. Both the area and intensity of BME were highest between 3 and 6 months (p < 0.01). At 6 months, 68% of patients had BME, which significantly decreased by 9 months (p < 0.05). No cases of anterior subluxation were reported.
There was no correlation between the level of remodeling or bone edema and patients’ subjective experiences. In a group of 13 patients with BME and signal intensity 30-50% above average, dynamic loading was not allowed before the next MRI. BME reduction was noted in 84% of these patients by 9 months post-surgery (p < 0.05). Those who did not follow recommendations achieved only a 20% improvement in SI by the 9-month follow-up. Nearly half of this group (6 patients) developed anterior tibial subluxation.
Conclusion
MRI observations show that the period between 3 and 6 months post-ACLR is critical, characterized by intense biological changes and hyper-intense signals from BME and the graft. Research indicates that graft signals impact the biomechanical properties of the ACL, rendering it mechanically weakest and most susceptible to elongation and injury. As knee homeostasis is not yet achieved, high-impact activities should be restricted. Regular MRI assessments can help prevent excessive acceleration and unnecessary delays in the training process, allowing athletes to progress effectively when both functional and structural readiness are attained. Appropriate decisions made during this phase may help reduce the risk of re-rupture.