This blog is inspired by the recent paper that Oliveira and colleagues published on “Medial patellofemoral ligament anatomy: Is it a predisposing factor for lateral patellar dislocation?” The study was published in International Orthopaedics in February 2014.
This study provides descriptive statistics of the length and insertional thickness of the medial patellofemoral ligament (MPFL) in normal, patients with pain and subluxation and patients who have had outright documented patella dislocations. The goal of the work was to begin to answer the important question if a previously injured and potentially elongated MPFL is potentially a risk factor for recurrent patella dislocations.
Through previous work, we have learned that certain elements of the bony anatomy of the knee joint is a potential risk factor for recurrent patella dislocation. This includes trochlear dysplasia (85% to 96%), an increase in the tibial tubercle–trochlear groove (TT-TG) distance, indicating lateralized insertion on the tibia relative to the trochlear groove (56%), patella alta (24%) and lateral patella tilt (83%). While the majority of these measurements are bony measurements, the lateral patellar tilt may be regarded as a composite of bony architecture as well as medial and lateral soft tissue balance. A true guideline for soft tissue integrity of the medial or lateral restraints that may very well play a role in patella instability is lacking.
This is why I consider the work of Oliveira and colleagues interesting and important. Clinically, we have learned from Sillanpaa and colleagues that patients with femoral MPFL injuries or mid-substance tears have a significantly higher redislocation rate than patients with patella avulsions when undergoing primary repairs. The thought is that it has to do with the predominant repair technique of “reefing” the MPFL toward the patella, which would elongate the ligament through the injured tissue (i.e., the mid-substance tear or femoral sided tear) when tension is applied on the patella sided insertion of the injured ligament. Subsequently, they have cautioned us to consider MPFL injury characteristics when performing MPFL repairs.
Until now, however, we did not know if this elongation of the MPFL actually happens. Oliveira’s paper confirms that this post-injury elongation exists and can be as long as 1.35 cm on average in patients who present with recurrent dislocation episodes. Interesting is that the difference in patients who do no report instability after primary dislocations is not significant.
The pure descriptive statistics of this paper indicate that the average length of the MPFL as measured on MRI is 4.9 cm ranging from 3.61 cm to 6.18 cm. They found that in patients with any trochlear dysplasia and lateral patellar tilt of more than 20° and no history of patella dislocations the MPFL was never longer than 6.0 cm. Therefore, they considered the length of 6.0 cm a maximal normal MPFL length.
Furthermore, in order to normalize the MPFL length against condylar size they calculated a ratio of the intercondylar width calculated on the axial MRI against the length of the MPFL [IC length (mm)/MPFL length (mm) = instability coefficient (IC)]. The normal value should be between 1.3 and 1.95. This value is lower with a longer MPFL. The authors considered the IC of greater than 1.3 as normal and less than 1.3 as potentially abnormal based upon their descriptive analysis.
While this paper does not identify or statistically prove true risk factors (the study is underpowered to do so), it provides some support to the belief that previous injury and rehabilitation of the dislocated patella may lead to an elongated MPFL. It also helps to put a preliminary number to a potentially critical length of the MPFL that may guide us in the decision making process of whether to operate on the MPFL (or reconstruct) in a soft tissue rebalancing or bony procedure for a patient with recurrent patella dislocations.
I think the analysis of potential risk factors for patella dislocation is important to be able to counsel patients better. Each patella dislocation event has the potential for significant additional harm and it is often a major setback for the usually young individual who has just worked so hard to return to previous activities. Any bit of information we can harness to prevent these redislocations without indiscriminately operating on everyone with a primary dislocation is important.
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Christian Lattermann, MD, is an associate professor, Vice Chairman for Orthopaedic Research, Director at the Center for Cartilage Repair and Restoration, Department of Orthopaedic Surgery and Sports Medicine, University of Kentucky. He can be reached at Department of Orthopaedic Surgery and Sports Medicine, University of Kentucky, 740 South Limestone, Lexington KY 40536; email: Christian.Lattermann@uky.edu.
Disclosure: Lattermann is a consultant for Sanofi/Genzyme and Isto and receives institutional support from Smith& Nephew.