Anterior Knee Problems after Anterior Cruciate Ligament Reconstruction

Patellar Tendon and Hamstring Tendon Autografts

Until recently, the gold standard in ACL reconstruction was mid-third bone–patellar tendon–bone (BPTB) autograft. However, it has been associated with anterior knee symptoms in up to 60% of patients. An increased incidence of anterior knee problems, such as pain and loss of sensation, was found in patients where a patellar tendon (PT) autograft was harvested from the healthy contralateral knee, suggesting that this procedure should be avoided because additional problems and morbidity are transferred onto the contralateral knee. As a consequence, the use of four-strand hamstring tendon graft has increased in popularity, since these problems seem to be less frequent.

There have been conflicting results when comparing these two most popular grafts, regarding the incidence of anterior knee pain. This variability in results is mostly attributable to inherent difficulties to achieve homogeneity owing to technical issues (method of graft harvest, preparation of the graft, cycling of the graft, degree of knee flexion and graft tension when securing the graft, fixation method, rehabilitation protocol, and outcome measures). Ideally, the incidence of anterior knee symptoms between different types of grafts can be compared through large randomized controlled trials, but such evidence is still scarce. However, recent meta-analyses concluded that ACL-reconstructed knees using PT autograft are more prone to develop anterior knee symptoms and extension deficits compared with hamstring tendon (HT) reconstructions.

Harvesting trauma, patellar tendinitis, tendon changes during the repair process of the tendon gap, vascular damage of the retropatellar fat pad, and proprioceptive loss of the extension mechanism have all been implicated in the pathophysiology of anterior knee pain following ACL reconstruction. Patellar tendon shortening is another important factor for developing anterior knee pain. Using magnetic resonance imaging (MRI) evaluation, it has been demonstrated that significant PT shortening (mean 9.7%) occurs after harvesting Bone–Patellar Tendon–Bone (BPTB) grafts, compared with the contralateral nonoperated knee 1 year after ACL reconstruction ( Fig. 136.1 ). A possible explanation for this is retropatellar fat pad fibrosis secondary to the surgical trauma, contraction of the scar that develops in the gap created after patellar tendon harvesting due to diminished elastic components, and the decreased strength of quadriceps contributing to patella baja, which stresses the patellofemoral joint. Moreover quadriceps inhibition/weakness causes delayed rehabilitation with subsequent extension deficit and abnormal patellofemoral joint forces. In contrast, harvesting the HTs resulted in a nonsignificant shortening of the patellar tendon of 2.6% ( Fig. 136.2 ). Using an Insall-Salvati ratio less than 0.74 as the MRI criterion for patella baja diagnosis, 12.5% of the patients in the BPTB group and 3% in the HT group were found to develop patella baja after surgery. However, this shortening was not of clinical importance, as it was not associated with anterior knee pain in the short-term follow-up. Another study with a longer follow-up has shown that the severity of patellofemoral joint arthritis and anterior knee symptoms correlate with the amount of patellar tendon shortening ( Fig. 136.3 ).

Central PT harvesting has been found to cause a slight medial displacement of the patella, and this alteration in position causes high contact forces in the medial patellofemoral joint. A solution to this may be the use of the medial third of the patellar tendon, which does not influence the patellofemoral angle and causes an insignificant lateral patellar displacement.

Anterior knee pain does occur after hamstring ACL reconstruction, although the anterior structures of the knee remain intact. The reason for this is not clear, but it is known that the patellofemoral joint can be the source of pain after nearly any surgery to the knee, even if the patellofemoral extension mechanism is not directly involved. The incidence of anterior knee pain after HT ACL reconstruction is less than 23%. Evaluation of the pain with diagrams has shown that it is more diffuse and is not related to the skin incision for tendon harvesting or tibial tunnel drilling. In contrast, anterior knee pain after PT harvesting is more localized, and palpation reveals trigger points usually over the inferior pole of patella, the tibial tuberosity, or above the patellar tendon donor site.

There are conflicting reports whether grafting the patella and tibial tunnel bone defects after BPTB harvesting reduces or does not reduce the incidence of anterior knee symptoms. There is also a report that patella grafting increases the incidence of painful spurs at the inferior pole of the patella. Similar arguments have been raised regarding whether suturing the patellar tendon gap facilitates tendon healing or is a cause of patellar tendon shortening.

Disruption of the extensor apparatus, although uncommon, has been associated with BPTB ACL reconstruction. Patellar fracture is unique in bone–PT–bone ACL reconstructions, and its incidence has been found to range between 0.2% and 2.3%. Patellar tendon rupture is another rare complication that may occur after PT harvest, with a reported incidence of approximately 0.24%. Tendon devascularization and alterations in the healing and remodeling process possibly predispose to this complication. The rupture usually occurs in either a proximal-medial to distal-lateral pattern or an entirely distal pattern, in contrast to proximal-only tears commonly observed in unharvested PTs.