Several factors have been identified as predisposing factors to CMC joint osteoarthritis. Hyperlaxity with subluxation of thumb CMC joint but also anatomical factors related to trapezium shape and obliquity. Articular joint shape and congruency have also been studied. The aim of this radiological an CT scan study is to define radiological parameters and particularly joint congruency that can be predisposing factor to CMC joint OA.

Forty-three hands (females 35, males 8) in 23 patients affected by CMC OA were studied with X-rays (AP, oblique, stress view, Robert's view) and CT scan. Mean age 62 (46–84). Ethic Committee approval was obtained. Control group were 52 normal hands (38 females and 14 males) in 26 patients, mean age 29 (25–40). Measured parameters: ratio between height of overall trapezium (TH max) and height of saddle (TH min) was measured (TH max/TH min and TH max–TH min/TH max); The saddle concavity (TH max–TH min) and the ratio between first metacarpal subluxation/width of trapezium (RS/AW); trapezium obliquity measured on X-rays as an angle with II metacarpal (alphaTT angle). Metacarpal subluxation was measured from centre of scaphoid-centre of metacarpal base distance on CT scan (SMD). These parameters were examined by two radiologists and 2 orthopaedic surgeons. Inter and intra-observer variability were measured by ICC index.

TH max–TH min in patients with OA was 2.63±1.7, while in normal patients was 1.52±0.5 this was statistically significant (P<0.01).

TH max/TH min: 1.29±0.2 while in control group was 1.13±0.05 (p<0.05).

TH max–TH min/TH max: 0.20±0.1, while in control group was 0.11±0.04 (P<0.01).

RS/AW 0.28±0.11 while in control group was 0.29±0.19 (non-significant).

AlphaTT 38.17°±11.6, in control group 33.10±6.23 (P<0.01).

SMD 18.17±3.21, in control group was 1.31±0.16 (non-significant).

Intra-observer ICC values were good or excellent and Inter-observer ICC values were excellent for all parameters.

The results show that saddle concavity and trapezium obliquity are increased in the pathological group compared to normal group and increase with the development of OA. The metacarpal subluxation is better evaluated with CT scan as it develops in three planes, on X-rays it was not significant this means that laxity is present either in normal and pathologic.

Defining predisposing factors to thumb CMC joint OA can be useful in order to prevent the disease and to treat the patients early. In the clinical practice, they can be useful particularly in planning trapezium reshaping in partial trapeziectomy or to improve implants design.