Figure 7.41 PP head bearing surface feature curve profiles of a selected curve from all specimens

The bearing surface of each finger joint specimen could be reconstructed using the detailed information obtained from the database. Since it is not practical to provide either a custom-made prosthesis for each joint or the range of sizes for the whole population, we must use a generalized model derived from statistical analysis. Also, it is impossible to have a uniform model for the whole population, so we must further classify the model according to the sizes of human finger joint. To meet the requirements from the whole population, five size classes are suggested after consultation with the designer, S. P. Chow. Therefore, the final design is categorized into five groups according to the radii of the best-fit circles, with a 0.5 mm size increment in the best-fit circle radius size. The radii of 2.5, 3.0, 3.5,4.0 and 4.5 mm are chosen as the typical geometry sizes.

Although the two condyles of the bi-condylar PP head are not symmetrical (Ash and Unsworth, 1997), the condyles of the reconstructed bearing surfaces are designed to be symmetrical for surgical convenience and economical considerations. The surfaces for the left and right hand are also constructed symmetrical. Therefore, the reconstructed anatomy of the joint is slightly different from the original joint structures; however, this modification will help to reduce the number of joint prostheses required for finger joint replacement surgery. At the same time, we show that such approximations for the original finger joint are acceptable in terms of geometry precision or shape tolerances.

Figure 7.42 shows the reconstructed PP head surface for the middle finger of specimen 5. The difference between the reconstructed surface model and the laser-scanned point cloud data is inspected (Figure 7.43). The inspection deviation analysis is as follows:

  • The maximum negative difference is 0.26 mm.
  • The maximum positive difference is 0.28 mm.
  • The average difference is 0.027 mm and the standard deviation is 0.091 mm.

Figure 7.44 shows the averaged symmetrical PP head surface reconstructed for all PP heads and Chow's finger joint replacement design of the PP head. Chow's design is the latest finger joint prosthesis designed by HKU and still subject to further modifications.

Figure 7.45 shows the reconstructed MP base surface for the index finger of specimen 8. The difference between the CAD reconstructed surface model and the original point cloud data shown is in Figure 7.46.

S 43p_tW£d a <HP_h»5d a MPJiesd fa S2FJ-BXI a 53P JWS3 a V-- ' -.'yj xF.ir_>,■■■:.! a62P_httd

K 63P_hMd a 64P_hesd

  • 7 IP_hisd
  • 73=_hejd a 73P_hejd a T-rjiead a RIP iwti
Figure 7.42 PP head surface reconstructed for the middle finger of specimen 5

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