All patients followed the same rehabilitation protocol [48]. The knee was kept in a brace locked in extension for 3 weeks, and patients were allowed partial weight bearing with crutches, followed by weight bearing as tolerated. A continuous passive motion machine was used for 4–6 h per day in a range between 20° and 60°, starting on the first postoperative day. The range of motion (ROM) was increased up to 90° by 2 weeks postoperatively and then gradually increased up to 120° of flexion thereafter. Full active ROM was achieved between 6 and 12 weeks after surgery. Running was allowed at 3 months. No contact sports were allowed before 5 months.

All patients were available at final 5-year follow-up. No infections or major postoperative complications were seen in this case series. Four patients (8 %) had a re-tear during sporting activity and underwent ACLR within 2 years from primary ACL repair; for these patients, the most recent evaluation score completed at 1-year follow-up, prior to revision surgery, was included in the final analysis.

The difference in anterior translation of the knee compared to the unaffected side was reduced from 4.1 mm preoperatively to 1.4 mm at 5-year follow-up (p < 0.05). A significant improvement in Tegner, single assessment numeric evaluation (SANE), Marx, Noyes, and Lysholm scores was observed at 5-year follow-up (p < 0.05). The final International Knee Documentation Committee (IKDC) objective score was rated as normal in 39 patients (78 %), nearly normal in 10 patients, and abnormal in 1 patient. The 11 patients with a nearly normal or abnormal IKDC score had associated pathologies (meniscal or chondral lesions). Thirty-nine patients (78 %) fully resumed sporting activity. Return to sport was reached at a mean of 6 months postoperatively. Eleven patients (22 %) did not return to sport at pre-injury levels; in four of these patients, this was a personal choice unrelated to functional ability.

Second-look arthroscopy was performed in six patients (12 %) and consistently revealed a healed ACL which was stable on probing and had minimal fibrous tissue contained within the healed ligament (Figs. 13.4 and 13.5).

ACL primary repair combined with bone marrow stimulation and growth factor application is an effective technique to restore knee stability and function in young athletes presenting with partial ACL tears [47]. Potential benefits include preservation of the native ACL and avoidance of complications associated with ACLR surgery, such as loss of proprioception. MSCs and PRP have the capacity to act as a source of precursor cells and growth factors that have been shown to enhance ligamentous healing [43, 44]. Anatomic repair and apposition of the torn ligament fibers are essential, as gapping between ligament fascicles may prevent cell migration and tissue regeneration [31].

The potential benefits of MSCs in ACL repair have also been described by Steadman et al., who reported excellent outcomes in terms of knee stability, function, and return to sport [20, 49]. The authors investigated the results of this procedure in the treatment of proximal ACL tears in a group of 48 active individuals over 40 years of age and reported improved clinical outcomes after a minimum of 2-year follow-up. In another study, excellent clinical outcomes were reported in 10 of 13 athletically active, skeletally immature patients with proximal ACL tears treated with a “healing response” procedure (ACL femoral footprint microfracture) [21]. Interestingly, this procedure was performed without concomitant suture of the ACL.

In the present study group, 98 % of patients presented at final follow-up with a normal or near normal IKDC objective score and a Tegner score comparable to pre-injury levels. Improvement in other instruments (Marx, Noyes) indicated good outcomes and recovery of stability and function similar to pre-injury assessments. Although there are previous studies that have reported high re-rupture rates of the ACL (approximately 50 %) following primary repair [16, 19], the re-rupture rate of the present study cohort was significantly lower (8 %) and is comparable to the results following ACL reconstruction at similar time points [50, 51]. It should be highlighted, however, that not all ACL lesions can be treated with this technique; patient selection is essential and strict inclusion criteria should be followed. The relatively low proportion of partial ACL ruptures identified in young athletic individuals, combined with the requirement for adherence to a strict rehabilitation protocol, leads to a low number of available patients. The precise selection criteria, patient adherence to the physiotherapy regimen, and regular follow-up may be contributing factors to the high success rates (90 %) demonstrated at midterm follow-up.

Undoubtedly, biologic augmentation techniques to assist tissue repair and regeneration will continue to improve. For example, the addition of PRP preparations to MSCs has been shown to assist with the formation of bioactive composites suitable for the healing of tissue defects in vivo, by acting as a source of both growth factors and “working cells” [53]. The application of multiple biologics that have the potential to act synergistically may play an important role in the progress of regenerative medicine. Furthermore, with greater advances in tissue engineering and molecular biology, the development of scaffold and cell-scaffold composite technology may offer interesting therapeutic options to augment ligamentous repair. There has been reported acceleration of ligament healing by enhancement of ACL cell viability, metabolic activity, and collagen synthesis following the use of PRP-scaffold composites in experimental ACL models [42]. The underlying premise is that while PRP/MSCs will act as the source of growth factors and precursor cells, the scaffold acts both as a matrix in the cellular process and as a biomechanical support following primary repair of the ACL. This would provide a secure environment for the regenerative cells, separating them from the effects of circulating plasmin within the joint space, which is known to inhibit the process of fibrin clot formation.

The natural history of partial ACL ruptures should be considered when undertaking surgical management of such injury. In the active patient who wishes to return to sport, there may be progressive laxity and increasing functional limitation associated with partial ACL injury [52], and surgical treatment may be preferable early in the course of management. Although selective reconstruction of the AM bundle in cases of partial ACL rupture has been shown to restore stability [54, 55], standard ACL single-bundle reconstruction has not been compared to selective AM bundle reconstruction to a great extent in the literature [56]. The technique of ACL repair with biologic augmentation that has been described in our cohort of patients who had suffered partial ACL injury has demonstrated comparable outcomes to those expected in cases of either selective AM bundle reconstruction or a standard single-bundle technique. There is need for further controlled comparative studies to examine outcomes between surgical management techniques in patients with partial ACL injury in order to develop appropriate treatment guidelines.