Tibial tubercle osteotomy in association with MPFL reconstruction: When is it really needed?
Recurrent lateral patellar dislocation is a debilitating knee condition that frequently involves young, active patients, significantly affecting their quality of life. The medial patellofemoral ligament (MPFL) has been identified as a primary restraint to lateral patellar dislocation and its injury a key contributor to the loss of patellar stability. Reconstruction of the MPFL has become an increasingly common technique for the treatment of recurrent lateral patellar instability in order to address the loss of this key restraint to lateral patellar translation.
Numerous anatomic factors have been shown to contribute to patellar instability, including trochlear dysplasia, patella alta, and increased tibial tubercle-trochlear groove (TT-TG) distance. Among these factors, the TT-TG distance and patellar height are most amenable to surgical correction, through the performance of a tibial tubercle osteotomy (TTO). A key question is at what point are these anatomical factors severe enough to require surgical correction. In other words, when is an isolated MPFL reconstruction insufficient to restore patellar stability?
Influence of TT-TG distance
Increased TT-TG distance has long been recognized as a contributing factor to patellar instability and numerous procedures have been described to address this condition, most commonly through medialization of the tibial tubercle with a TTO. The classic teaching of the Lyon School in France - the “menu à la carte” - is that a TT-TG distance less than 15 mm is normal, while a distance greater than 20 mm is indication for tubercle medialization in symptomatic patients.
Robert A. Magnussen
While this teaching serves as an excellent guide of normal anatomy, one must carefully consider that the classic surgical menu does not consider the influence of MPFL reconstruction as its development predates this surgical option. The proximal soft tissue procedures at that time included VMO plasty and advancements, with the goal of soft tissue balancing and treatment of patellar tilt. These procedures function quite differently from an MPFL reconstruction and this difference may be important. Further, the performance of a TTO in addition to an MPFL reconstruction significantly increases the complication risk of the procedure and alters the rehabilitation protocol. It is therefore critical to establish when MPFL reconstruction alone is sufficient, avoiding the increased morbidity of the addition of a TTO.
There are few outcome studies of MPFL reconstructions regarding the influence of TT-TG distance on isolated MPFL reconstruction; however, several recent studies have demonstrated little influence of TT-TG on the outcome of MPFL reconstruction. Most notably, Matsushita et al this year demonstrated in a retrospective cohort study that isolated MPFL reconstructions performed in the setting of a TT-TG greater than 20 yielded similar Lysholm and Kujala scores to those performed with a TT-TG under 20. They noted no recurrent dislocations in either group, but the study is likely underpowered to detect significant differences in recurrence. Similarly, Wagner et al in 2013 reported no difference in Kujala scores based on TT-TG, but this study was underpowered.
It is important to note that in neither paper did the authors discuss in detail their indications for an isolated MPFL reconstruction in regards to TT-TG distance, although Wagner et al did recommend consideration of a TTO if the distance exceeds 20 mm. There likely are some patients with excessively large TT-TG in whom a TTO is indicated - what is unclear at this point is exactly where that cutoff should be and if that cutoff is absolute or influenced by other anatomical factors. More research in this area is critical
The literature clearly demonstrates that the TT-TG distance alone (at least at this point) does not completely inform the decision as to whether a tibial tubercle medialization is indicated. One must consider other factors including physical examination, history, and patient preference when making such a decision. Although no consensus exists in the literature, certain physical examination findings including a prominent J-sign, lateral patellar tracking, and generalized ligamentous laxity may portend worse results of isolated soft tissue procedures and push one toward an osteotomy in cases of borderline TT-TG. Other factors, including varus knee alignment or external tibial torsion are cited as contraindications to medialization. Aspects of the patient history, including the circumstances surrounding the initial and recurrent dislocation episodes (traumatic versus atraumatic, bilaterality, etc...), activity level, and expectations are also important and should be carefully considered.
Influence of patellar height
Patella alta has been cited as a contributing factor to recurrent patellar instability and patella alta with a Caton-Deschamps (C-D) index greater than 1.2 has classically described as an indication for tibial tubercle distalization osteotomy. As with the TT-TG distance as described above, this value was settled upon in the absence of an MPFL reconstruction; however, significant untreated patella alta has been implicated as a potential reason for failure of isolated MPFL reconstruction. The study by Wagner et al referenced above also assessed the influence of patella alta on outcomes of isolated MPFL reconstruction and noted no significant difference, although the study did not include many patients with significant alta and is underpowered.
As with the TT-TG, there certainly exists a degree of patella alta above which an isolated MPFL reconstruction will fail to restore patellar stability. What remains unknown is whether the C-D index of 1.2 is an appropriate cutoff point or even if one such cutoff point exists for all patients. An interesting concept to consider when treating patellar instability is the interplay between patella alta and trochlear dysplasia. Patella alta is theorized to contribute to patellar instability in part by delaying entry of the patella into the trochlear groove as the knee flexes, therefore requiring less force to laterally displace the patella. Similarly, in the setting of trochlear dysplasia, a shallow trochlear groove leads to less bony constraint of the patella, particularly in the superior portions of the groove that are more involved in lower grade dysplasia. Because trochlear dysplasia and patella alta decrease patellar stability by similar mechanisms, there is a clear interaction between the two and a patient with both patellar alta and dysplasia is at higher risk for instability than a patient exhibiting either in isolation.
It follows that when considering adding a tibial tubercle distalization to an MPFL reconstruction, one much consider not only the patellar height, but also the degree of trochlear dysplasia that is present. A physical examination test that can help explore the effect of this interaction and aid in surgical decision-making for individual patients is the patellar apprehension and relief test. Patellar apprehension has been widely discussed, but equally important is the degree of knee flexion above which the apprehension disappears. The more severe the patella alta and trochlear dysplasia, the greater degree of knee flexion is required to relieve apprehension. Apprehension that is relieved at 30° to 40° of flexion suggests a good chance of restoring patellar stability with an isolated MPFL reconstruction, while persistent instability beyond 45° or especially 60° of knee flexion suggests that significant patella alta, trochlear dysplasia, or both exist. Patients with significant patella alta (C-D index greater than 1.3) and delayed relief of apprehension with knee flexion are excellent candidates for distalization.
As patella distalization is performed much less frequently than medialization currently, it is important to discuss a few key points when considering this procedure. After identifying an appropriate patient, one must determine the degree of distalization needed. It is important to note than a restoration of patellar height to a C- index of 1.0 is generally not required - particularly given the disastrous consequences of patella infera that can result from over-aggressive distalization. Generally a target C-D index near 1.1 is reasonable in this author’s experience. On must also remember that due to the anatomy of the proximal tibia, a straight distalization will also result in a decrease in the TT-TG in most cases. Aggressive combined medialization and distalization can result in effective over-medialization of the tubercle and iatrogenic medial patellar subluxation or dislocations as well as potentially increased risk of arthritis due to increased contact pressures.
The indications for the addition of a tibial tubercle osteotomy to a MPFL reconstruction are not completely clear at this point. One must consider patient and physical examination factors and the increased morbidity of a TTO in addition to key radiographic measures when making this importance treatment decision.
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Robert A. Magnussen, MD, MPH, is an assistant professor of Clinical Orthopaedics; Team Physician, The OSU Sports Medicine Center, Ohio State University, Columbus, Ohio.
Disclsoure: Magnussen has no relevant financial disclosures.