First-time patellar dislocation typically occurs with twisting knee motions, during which the medial ligamentous stabilizers rupture, and the patella strikes against the lateral femoral condyle. The typical injury pattern is a tear of the medial patellofemoral ligament (MPFL) and bone bruises of the patella and the lateral femoral condyle. Additionally, complex injuries to bone, cartilage, and ligaments may occur. The ensuing loss of medial restraint favors future patellar dislocations, especially if additional risk factors are present. Recurrent patellar dislocations usually occur in individuals with anatomic variants of the patellar stabilizers, such as trochlear dysplasia, patella alta, and lateralization of the tibial tuberosity. Magnetic resonance (MR) imaging is reliable in identifying risk factors for chronic patellar instability and in assessing knee joint damage associated with patellar dislocation. MR imaging can thus provide important information for individually tailored treatment. Patients with primary patellar dislocation without severe internal derangement who lack major risk factors can be treated conservatively. Patients with pronounced ligamentous tears or large osteochondral lesions require prompt surgery. In addition, surgical correction of anatomic variants will help reduce the potential for chronic instability. The most common procedures, in addition to MPFL reconstruction, include trochleoplasty, medialization of the tibial tuberosity, and medial capsular plication. For comprehensive assessment of patellar dislocation, a radiologist should be able to identify typical injury patterns, know standard methods to assess risk factors for patellar instability, and be familiar with surgical options.

After reconstruction of the cruciate ligaments, replacement grafts have to undergo several phases of healing in the intra-articular graft region and at the site of graft-to-bone incorporation. The changes in the biological and mechanical properties of the healing graft in its intra-articular region are described as the ligamentization process. Significant knowledge has been added in the understanding of the several processes during the course of graft healing and is summarized in this article. The understanding of the spatial and time-dependent changes as well as the differences between the different models of graft healing are of significant importance to develop strategies of improved treatment options in cruciate ligament surgery, so that full restoration of function and mechanical strength of the intact cruciate ligaments will be achieved.

Purpose A summary is provided on the existing knowledge about the specific healing phases of the intra-articular hamstring tendon graft used for ACL reconstruction. Differences between human and animal in vivo studies are explained, and implications for the postoperative time period are laid out.MethodsA systematic review of the existing literature was performed on the topic of tendon remodelling of hamstring grafts in ACL reconstruction using Medline database. Publications between 1982 and 2012 were included. Special focus was directed on in vivo human and animal studies analysing intra-articular free tendon graft remodelling.ResultsAnimal and human in vitro and vivo researches have demonstrated three characteristic stages of graft healing after ACL reconstruction: an early graft healing phase with central graft necrosis and hypocellularity and no detectable revascularization of the graft tissue, followed by a phase of proliferation, the time of most intensive remodelling and revascularization and finally, a ligamentization phase with characteristic restructuring of the graft towards the properties of the intact ACL. However, a full restoration of either the biological or biomechanical properties of the intact ACL is not achieved.ConclusionSignificant knowledge on human cruciate ligament remodelling has been added in the understanding of the processes during the course of graft healing. Most importantly, the remodelling process in humans is prolonged compared to animal studies. While today´s rehabilitation protocols are often extrapolated from findings of animal in vivo healing studies, current findings of human in vivo healing studies might require new post-operative regimens following hamstring ACL reconstruction.

Our results suggest that some patients with nontraumatic patellar instability have greater internal femoral rotation, greater knee rotation, and a tendency for genu valgum compared with healthy controls. Rotational malalignment may be a primary risk factor in patellar dislocation that has so far been underestimated.

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