A novel in vivo skin extensibility test for joint hypermobility

The stress/strain curve derived from stretching skin is not linear, but follows a J-shaped curve. An initial generous yield is followed by a steep linear phase where considerable additional force is required to achieve modest increases in deformation. The former represents the taking up of slack resulting from the alignment of dermal collagen bundles in the line of force, while the gradient of the latter represents Young's modulus for skin. Skin hyperextensibility in Ehlers-Danlos syndrome (EDS) is limited to the initial phase of taking up slack. Skin hyperextensibility and joint hypermobility (JHM) form part of the Revised 1998 Brighton diagnostic criteria for the benign joint hypermobility syndrome (BJHS), considered by many to be akin to EDS-hypermobility type. JHM may be screened for using the Beighton Score or a 5-point questionnaire. Our aim was to validate a novel method of measuring skin extensibility based on these observations in addition to revalidating the 5-point questionnaire.

250 volunteers (131 female), median age 39 years (range 18-89 yrs), without BJHS, had their joint mobility evaluated using the Beighton Score, compared to the 5-point questionnaire. A Beighton score > or = 4/9 was considered to represent JHM. Skin extensibility was determined by placing 2 dots on the dorsum of the right hand between the second and third metacarpals, approximately 10 mm apart, and was measured using an electronic caliper. Perpendicular to the metacarpals, a force was applied until the skin was fully taut and the increment was measured. Skin-fold thickness was measured using a Harpenden caliper. A corrected skin extensibility score (CSES) was calculated by dividing the percentage increment by skin thickness. Interobserver variability was measured in a further 50 healthy volunteers.

The prevalence of JHM was 17.6%. Revalidation of the 5-point questionnaire returned a sensitivity of 0.85 and specificity of 0.85. The mean CSES was 23.84%/mm in the hypermobile group versus 13.55%/mm in the normal mobility group (p < 0.0001). CSES sensitivity was 0.72, specificity 0.75. The kappa value for interobserver variability was 0.83.

The CSES is a useful and reproducible measure of skin extensibility in health. Further work is warranted to validate this test in patients with BJHS.