Study of Finger Joint Morphology for Implant Design

A series of implant designs were generated during development of this new artificial PIP joint with progressive modifications and improvements over previous models [26]. Anatomical compatibility was given priority since it is to be used with the natural bony and ligamentous structures. Most recent artificial knee joints focus on anatomical restoration in contrast with older designs using a fixed hinge. Recent finger prostheses follow the trend by employing substantial consideration of the functional anatomy of both the IP and MCP joints, with minimal bone excision, the modeling of true anatomical articular surface as well as corresponding soft-tissue balance incorporated into the design [27, 28].

Morphometrical data directly from real phalangeal bones were measured to provide a basis for an anatomical implant design. Similar to total condylar knee prostheses [29], geometric parameters were established for the design of different functional parts of the artificial PIP joint with reference to previous works on Caucasian finger joints [30-31]. These functional parts for the joint design are illustrated in Fig. 9.2 and include (1) major sizes of proximal and distal implant components, (2) sizes of fixation stems on proximal and distal components, (3) coverage of joint curvatures at head of proximal phalanx and base of middle phalanx for the design of implant ROM, (4) contour of articulating surfaces for the design of implant bearing surface and (5) backing surfaces of proximal and distal components which will be supported by resected bony bed.

The radial and ulnar condyles on the head of the proximal phalanxes exhibit asymmetry on different digits [31] favoring convergence of the digits towards the palm. Inter-digital variation of skeletal structure among population compounds the difficulty to unify a design suitable for all fingers. 8 different PIP joint designs are required for the index, long, ring and small fingers on both the left and right hands. If 5 sizes of each joint implant to accommodate variations in population, then there are at least 40 implants and associated surgical instrumentation needed for a complete implant system for the PIP joint. All current artificial finger joint systems do not consider condylar asymmetry and a

Fig. 9.2 Functional elements of artificial finger joint design

1 Fixation stem

Fig. 9.2 Functional elements of artificial finger joint design common joint design compromise for PIP joints on all digits is applied in clinical practice.

Ten embalmed Asian hand specimens with a mixed gender were used to study the morphological characteristics on phalangeal bones. The proximal and middle phalanxes from index, long, ring and small fingers were cleared of soft-tissue attachments. 80 phalangeal bones were then mounted in angle holders for x-ray imaging. Basic bone dimensions denoting the bone length 'Lpp', the head diameter 'Dh', the head width 'Wh' of the proximal phalanxes and the bone length 'Lmp', the base height 'Hb', the base width 'Wb' of the middle phalanxes as indicated in Fig. 9.3, were measured by an engineering CAD system.

Preferably, one physical quantity of the phalangeal bone should be chosen as a common denominator for the best fitting implant for all digits. All other dimensions can then be defined by scaling to this denominator, which should be easily obtained through direct or x-ray examination to facilitate preoperative planning.

Size matching for some finger joint implants is based on best fitting of the stem to the space of the medullary cavity of the bone shaft. Mismatched implant size is suggested to have inferior effects on the active ROM due to an erratic joint rotational centre and a changed flexor moment arm [32, 33]. Recent studies recommend the selection of implant type to enable the restoration of the original joint rotational centre and a best-fit head diameter of the proximal phalanx in the sagittal plane [31]. Examination of the head diameter 'Dh' was undertaken as the possible denominator for the implant design. Although variations exist among different digits, the mean head diameter from the mixed group showed no significant difference for all individual digits. A single implant design generated from Dh as denominator can be applied to all digits excluding the thumb with 99% level of confidence. Dh from the mixed group

A-P view

Fig. 9.3 Major dimensions of phalangeal bones

A-P view

Fig. 9.3 Major dimensions of phalangeal bones ranged from 5.14 to 9.12 mm and followed a normal distribution with prominent sizes of 7 and 7.5 mm representing 40% of the sample population.

Measurements of the head width 'Wh' of the proximal phalanx, the base height 'Hb' and the base width 'Wb' revealed a pattern comparable to the bone lengths. The basic joint dimensions have a strong dependency on joint size which in turn correlates with the head diameter. Statistical analyses revealed good consistency and no significant difference of the Wh/Dh, Hb/Dh or Wb/ Dh ratios amongst the mixed group. This essentially provides Dh as a single parametric quantity that enables a common design suitable for all digits to be established.

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