Design Objectives for a New Artificial Finger Joint

The objectives are to develop a new finger joint prosthesis that provides:

1. an anatomical structure with intrinsic joint stability to reproduce joint balance and a near normal range of motion,

2. stable anchoring at the bone-implant interface by press-fit mechanical interlocking and long-term fixation to bone by osseointegration, and

3. high wear resistance through the use of advanced materials.

To integrate these novel ideas, the following issues had to be solved:

1. The linkage between the modular connection of joint components and the fixation to bone was considered.

2. A critical design of the articulating surface which can reproduce the normal range of motion and joint stability with sufficient thickness to withstand wear and tear long-term was developed.

3. The right materials and the manufacturing process were determined to produce a reliable and durable end product.

4. A testing protocol for the preclinical evaluation of artificial finger joint was developed for the examination of the performance of the new joint design in accordance with specifications.

All implant systems should restore original function with minimal side effect to the implanted body. Synthetic biomaterials face problems of fatigue failure and degradation which doesn't match the healing and regeneration processes of living tissue. Implantation triggers responses from the immune system attacking both healthy tissues and implant. Furthermore, pathological disturbance by surgical intervention compounds the difficulty of functional restoration. Operative techniques are still inadequate in providing fully recovered function. This strongly governs the effectiveness particularly in the case of orthopaedic implants where joint stability, mobility and deformity correction are of prior concern.

Early finger joint designs focused on functional features such as resembled ROM and joint stability. Newer designs emphasize restoration of functional anatomy and bone implant adaptation. Any new design must follow 3 main requirements: (1) Anatomical compatibility, (2) Functional compatibility and (3) Materials compatibility. Mechanical hinge joints maximize the stability of a finger joint without collateral ligament support but loosen due to poor bone-implant adaptation. Silastic implants have minimal constraint on bone but face problems of materials fracture. Unconstrained prostheses provide the best functional ROM but joint stability is insufficient and high recurrent deformities persist. It is suggested that there be a good compromise among all design criteria since all issues on implant design cannot be fully achieved as illustrated in Fig. 9.1.

Criteria for the new design of artificial PIP joint should conform to: Anatomical compatibility including:

• minimal resection to bone thereby maintaining the joint integrity with the supportive soft-tissue structure;

• minimal invasive surgery thereby inducing fewer traumas to the tendon network essential for joint mobility;

Fig. 9.1 Compromised design considerations for ideal joint implant system

• resemble the diameter of the natural joint in lateral direction maintaining a proper moment arm and mechanical advantage;

• maintain joint space and the original bone length of the replaced joint;

• fit into the internal medullary canal of phalangeal bones thereby achieving press-fit macro-interlocking fixation;

• a large variety of implant size and stem size thereby providing a coherent bone-implant adaptation;

Functional compatibility including:

• withstand loading under physiological conditions and effectively transmit joint load to the bony bed underneath;

• unconstrained articulation thereby promoting recovered ROM;

• intrinsic stability from the unconstrained articulation thereby enriching the support from attenuated collateral ligaments;

• proper alignment to the respective phalangeal bones;

• resist dislocation force thereby correcting deformity to improve cosmetic appearance;

Material compatibility including:

• biocompatibility;

• sufficient strength to bear loading without permanent failure;

• superior wear and tear properties thereby improving the service life;

• ability to be manufactured into a delicate shape.

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