Sexual Dimorphism in Bones of the Thenar and Hypothenar Aspects of the Hand

Osteoarthritis of the pollical carpometacarpal joint is significantly more prevalent in females than in males. Although a number of hypotheses to explain sex differences in the prevalence of thumb osteoarthritis have been proposed, one possibility is that it is a product of sexual dimorphism where female hands have smaller joint surfaces relative to muscle strength, which results in higher stresses on the joints. Although dimorphism of the skull and long bones has been extensively researched, there are few basic data on female‐versus‐male differences in the short and irregular bones. Therefore, it is unclear whether sexual dimorphism is a relevant avenue of investigation for determining the cause of such sex‐based differences in hand pathology. Because powerful precision gripping is dependent on the thenar and hypothenar muscles, the hand bones with muscle attachments or that are directly subjected to transarticular loading by these muscles are of particular interest. Therefore, we investigated four hand bones contributing to the first and fifth carpometacarpal articulations in a sample of humans of African and European descent, and it was predicted that females would have smaller overall bone volumes and areas, smaller individual joint surface areas, as well as different joint and muscle attachment shapes. The hamate, trapezium, first metacarpal (Mc1), and fifth metacarpal (Mc5) of each individual were laser scanned to generate 3D surface polygon models. Volume and surface area were calculated for each bone as a whole in addition to the areas of specific joint surfaces. The shape of the pollical carpometacarpal joint (proximal Mc1 and distal trapezium) was assessed with quadric fitting functions to determine their dorsovolar and radioulnar curvatures. For the hamate, the orientation of the triquetrohamate joint surface relative to the proximodistal axis was quantified, and for the hamulus (which provides attachments for the flexor digiti minimi brevis, opponens digiti minimi, and flexor carpi ulnaris) its length, width, breadth, and angle were measured. The results indicate that females tend to have bones that are significantly smaller in overall size with correspondingly smaller joint surfaces, and a hamulus that differs in both size and shape. Future work will diversify the sample and incorporate measurements of relative thenar and hypothenar muscle force potential to further evaluate the biomechanical consequences of skeletal dimorphism. Such an approach may allow us to better investigate more specific factors contributing to the increased osteoarthritis risk in the hands of females. Support or Funding Information National Science Foundation (BCS‐1539741, BCS‐1317047), Wenner‐Gren Foundation, Leakey Foundation