BLOG: Form follows function with the patella
by Pieter Erasmus, MD
John P. Fulkerson, MD, recently wrote a blog article of his team’s findings looking at 3D reconstructions of the patellofemoral articulation in patients with trochlea dysplasia. Their analysis suggest the patella and trochlea are generally congruous in trochlea dysplasia, albeit malformed.
These findings add to a growing body of literature that supports the causal relationship between femoral condylar morphology and the patella. This suggests the principle of form follows function also hold truth for the patellofemoral articulation.
Patella alta and in-utero breech position have been linked to the development of trochlear dysplasia. Panni and colleagues found an association between Dejour type C trochlear dysplasia and Wiberg type III patella — both the patella and the trochlea have flat surfaces that fit one another. Salzmann and colleagues published a case report of abnormal trochlea development in a child, following a below knee amputation as an infant. In an animal study, Li and colleagues found trochlear dysplasia was a common occurrence following patellar dislocations.
The morphology and alignment of the patellofemoral joint (PFJ) are functions of genetic and epigenetic factors that play a role in the eventual shape of the joint. Kapandji postulated the morphology of the femoral condyles can be traced back to basic joint physiology, in which the shape of the femoral condyles is the result of pressure applied by the patella throughout development.
We would suggest form follows function in the PFJ and genetic factors play an indirect role in the development of the PFJ by affecting the general alignment of the whole lower extremity and its muscle attachments. The developing patella aligns itself in the soft tissue sleeve of the extensor mechanism, and not to the distal femur. The soft tissue sleeve is attached proximally to the superior and inferior iliac spines and the proximal femur and distally to the tibial tubercle. Its position and function are thus influenced by the alignment of the limb in the sagittal, coronal and axial planes. In this way, the resultant vectors acting on the immature patella and trochlea might play a causal role in the final morphology.
Changing the shape of naturally developed trochlea without considering the shape of its corresponding patella might lead to a misfit between the patella articulating surface and that of the reshaped trochlea. There is a possibility this might lead to early patella femoral degeneration. It might be more feasible, whenever possible, to stabilize the patella in its original position without disturbing the naturally developed congruous surfaces.
Fulkerson JP. J Sports Med Arthrosc Rev. 2019;doi:10.1097/JSA.0000000000000261.
Hvid I, et al. Acta Orthop Scand. 1983;doi:10.3109/17453678308992875.
Kapandji IA. The physiology of the joints: annotated diagrams of the mechanics of the human joints. 1970. Edinburgh: Livingstone, Edinburgh.
Li W, et al. Knee. 2013;doi:10.1016/j.knee.2013.05.016.
Øye C, et al. Acta Orthop. 2016;doi:
Panni AS, et al. Knee Surgery, Sport Traumatol Arthrosc. 2011 doi:10.1007/s00167-010-1329-4.
Rush J, et al. J Sports Med Arthrosc Rev. 2019;doi:10.1097/JSA.0000000000000254.
Salzmann GM, et al. Int J Surg Case Rep. 2014;doi:10.1016/j.ijscr.2013.12.021.
Disclosure: Erasmus reports no relevant financial disclosures.