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BLOG: Anisometry of MPFL reconstruction in the setting of patella alta, increased TT-TG distance
The incidence of patellar dislocation is 29 per 100,000 in adolescents and recurrence rates with nonoperative treatment have been reported from 15% to 44%. Dejour and colleagues identified four risk factors that contribute to recurrent patellar instability, including trochlear dysplasia (present in 85% of patients with patellar instability), increased patellar tilt (83%), patella alta (24%) and increased tibial tubercle-trochlear groove distance greater than 20 mm (56%) as compared with the presence of these factors in control knees of 3% to 6.5%. Of these factors, patella alta and increased tibial tubercle-trochlear groove distance are easily modifiable with a tibial tubercle osteotomy.
The medial patellofemoral ligament (MPFL) is the primary stabilizer to lateral patellar translation. MPFL tears are the essential lesion in patellar instability and thus, MPFL reconstruction has become the standard of care for treatment of recurrent patellofemoral instability. MPFL reconstruction allows most patients the ability to return to sports activities.
It has previously been shown that the MPFL is isometric through an early range of motion. Currently, there is a paucity of information regarding the nature of isometry of the reconstructed ligament in the setting of a lateralized tibial tubercle (increased tibial tubercle-trochlear groove [TT-TG]), patella alta (as measured by the Caton-Deschamps Index or CDI) or a combination of the two. It is unknown whether, in the setting of patella alta and/or increased TT-TG, if it is necessary to transfer the tibial tubercle in order to attain an isometric MPFL reconstruction and thus a stable patella.
Our group recently presented this study on April 14, 2016 at the Arthroscopy Association of North America Annual Meeting. We developed a custom, cadaveric testing rig to investigate the isometry of a reconstructed MPFL in various combinations of increased TT-TG and patella alta. A suture anchor was placed on the medial border of the patella and shuttled through the femur at Schottle’s point. Retroreflective markers were placed on the cadaver as well as on the suture to measure suture change in length through a range of motion. Suture motion less than 2 mm was considered isometric.
Increased TT-TG and patella alta significantly altered MPFL isometry, with suture motion up to 8 mm in certain conditions. Our findings suggested the threshold for isometry with respect to TT-TG likely lies somewhere between native and 20 mm. With the presence of patella alta, there was an interaction effect that dramatically increased the anisometry seen. The combination of TT-TG 20 mm and a CDI of 1.2 showed increased anisometry (mean maximal length change 2.94 mm) compared to TT-TG 20 mm alone (mean maximal length change 2.61 mm). When alta is increased to a CDI of 1.4, the mean maximal length change increased further to 3.92 mm. A mean maximal length change of 2.97 mm at a TT-TG of 25 mm alone suggests a higher TT-TG will affect anisometry. Similar to the findings within TT-TG of 20mm, there also appeared to be an interaction effect within TT-TG of 25 mm, given the increased mean maximal length change in the presence of patella alta with CDI of 1.2 (4.06 mm) and when increased to a CDI of 1.4 (4.72 mm). Given patella alta with a CDI of 1.4 combined with native TT-TG had worse anisometry (mean maximal length change 3.94 mm) than any alta state of TT-TG 20 mm (2.61 mm at native, 2.94 mm at 1.2 and 3.92 mm at 1.4), patellar height may have a greater influence on MPFL isometry than an elevated TT-TG.
Based on the parameters of our study and the settings we could control, we believe we have identified an appropriate set of conditions in which an osteotomy should be considered. Bony realignment, in the form of a tibial tubercle transfer, should be considered when performing an MPFL reconstruction for recurrent patellofemoral instability in the setting of increasing TT-TG and patella alta, with alta potentially being the more important variable, but especially when both are present, to prevent potential graft failure due to the anisometry demonstrated.
Reference:
Redler LH, et al. Arthroscopy. 2016;doi: http://dx.doi.org/10.1016/j.arthro.2016.03.054.
Lauren H. Redler, MD, is an assistant professor in the Department of Orthopedic Surgery at Columbia University Medical Center in New York, NY. She specializes in adult and pediatric sports medicine with a particular interest in patellofemoral disorders. She can be reached at lauren.redler@columbia.edu. Beth E. Shubin Stein, MD, is an associate professor at Weill Cornell Medical College and an orthopedic surgeon in the Department of Orthopedic Surgery & Sport Medicine at Hospital for Special Surgery in New York City.
Disclosures: Redler reports no relevant financial disclosures. Shubin Stein reports she is a consultant for Arthrex.