Abstract
We used CAD/CAM to compare the dimensions of patella tendon bearing (PTB) prostheses and total surface bearing (TSB) prostheses, and to assess how well they fit with the residual limbs, in order to standardize dimensions for automated production.
The study involved 17 below-knee amputees, aged 21-73 years. Two of the amputees had a short stump, twelve had a medium-length stump and three had a long stump. We used CAD/CAM to measure the stumps, and the PTB positive models and TSB positive models, and then made three-dimensional simulation images in order to compare them.
We measured the stump length, the antero-posterior diameter, the side-side diameter and the circumference each at the level of the patella tendon, the volume of the stump from the mid-level of the patella tendon and the shape at the mid-level of the patella tendon. We used the dimensions to calculate standardized PTB and TSB sockets. Next, using Student's t test, we compared the standardized PTB and TSB sockets with the stump length, activity, sex and age of the amputees.
Both sockets were significantly (p<0.05) longer than the stump length by about 2.5-7% and there was also a significant difference (p<0.05) between the lengths of the PTB socket and the TSB socket. On comparing the antero-posterior diameter and also the circumference at the level of the patella tendon, both sockets were significantly (p<0.01) smaller than the stumps by about 2.5-5%. In order to shed more light on the characteristics of the sockets, we calculated the volume of both the top half of the stump (measured from the patella tendon) and the bottom half of the stump. As for the top half, the PTB was about 5% smaller than the stump and as for the bottom half, TSB was about 4% smaller than the stump. This implies that in the case of the PTB, strong compression tends to be in the top half of the stump and in the case of the TSB it tends to be distributed in the bottom half of the stump. For both standardized sockets, a smaller volume tends to be better for increased activity. Objective assessment of amputees' stumps and prosthetic sockets using CAD/CAM can be used as the most suitable socket index.
