The patella is a sesamoid bone that typically doesn't completely from and ossify until ~3-6 years of age (e.g., Source). My long-standing understanding (supported by a claim in Saladin's college A&P textbook1 sitting on my shelf) was that sesamoid bones form "in response to stress."

Well, I had a student in my anatomy class this morning ask me the titular question about people born with leg paralyses or other congenital issues preventing them form walking. To their point, if such congenitally paralyzed people never walk and therefore never place stress on the knee via the quad tendon, would they ever form a sesamoid bone (i.e., the patella) in the knee?

I've done some digging and found evidence of "nail patella syndrome" (e.g., here) which results in people not forming a complete patella. However, I was unable to quickly narrow down any sources discussing lack of patellar growth due to congenital leg paralysis.

1. Saladin, K. S. 2015. Anatomy & Physiology: The Unity of form and function. 7th edition. McGrawHill, Education, New York, NY


In mice, patella would form, but not separate from femur.

On the development of the patella:

Previous studies have suggested a central role for mechanical load in the initiation of sesamoid development.

Our finding that the patella initially forms as part of the femur and is later separated by joint formation led us to revisit the issue of mechanical contribution to patella development. To achieve this, we studied patella development in mutant embryos devoid of contracting muscles, as a result of the naturally occurring autosomal recessive mutation muscular dysgenesis (mdg; Cacna1s). These mice lack excitation-contraction coupling and, consequently, their skeletal muscles lack contractility.

Histological analysis of sagittal sections of hindlimbs identified at E13.5, in both mutant and control littermates, the existence of patella progenitors, which were part of the femur. However, at E14.5, whereas in the control we observed joint formation between the patella and the femur, in mdg mutants this process was not detected. Consequently, at E16.5 the mutant patella was still part of the femur. These results suggest that, although muscle contraction does not affect patella initiation, it plays a crucial role later during the separation phase.

I know, you asked for human, but it is the only relevant information I found. However, while sources mention a role of physical stress in the calcification of sesamoid bones, the preceding formation of cartilage seems unaffected. Now, given the information that ossification occurs in mice without physical stress, I personally would conclude that the human patella would also calcify in paralyzed persons. Strictly, the question whether the patella forms, should be answered, if a non-detached patella counts.

  • $\begingroup$ Very good find! Thank you for this! +1. I will hold off accepting your answer, though, to see if anyone ever stumbles across any info regarding patella detachment, especially in humans. (I would have never thought such a topic was so difficult to research!) $\endgroup$ Apr 2 at 1:45

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