Design of a non-constrained, non-cemented, modular, metacarpophalangeal prosthesis

A Biomechanical Analysis of 2 Constructs for Metacarpal Spiral Fracture Fixation in a Cadaver Model: 2 Large Screws Versus 3 Small Screws

PURPOSE

Surgeons confronted with a long spiral metacarpal fracture may choose to fix it solely with lagged screws. A biomechanical analysis of a metacarpal spiral fracture model was performed to determine whether 3 1.5-mm screws or 2 2.0-mm screws provided more stability during bending and torsional loading.

METHODS

Second and third metacarpals were harvested from 12 matched pairs of fresh-frozen cadaveric hands and spiral fractures were created. One specimen from each matched pair was fixed with 2 2.0-mm lagged screws whereas the other was fixed with 3 1.5-mm lagged screws. Nine pairs underwent combined cyclic cantilever bending and axial compressive loading followed by loading to failure. Nine additional pairs were subjected to cyclic external rotation while under a constant axial compressive load and were subsequently externally rotated to failure under a constant axial compressive load. Paired t tests were used to compare cyclic creep, stiffness, displacement, rotation, and peak load levels.

RESULTS

Average failure torque for all specimens was 7.2 ± 1.7 Nm. In cyclic torsional testing, the group with 2 screws exhibited significantly less rotational creep than the one with 3 screws. A single specimen in the group with 2 screws failed before cyclic bending tests were completed. No other significant differences were found between test groups during torsional or bending tests.

CONCLUSIONS

Both constructs were biomechanically similar except that the construct with 2 screws displayed significantly less loosening during torsional cyclic loading, although the difference was small and may not be clinically meaningful.

CLINICAL RELEVANCE

Because we found no obvious biomechanical advantage to using 3 1.5-mm lagged screws to fix long spiral metacarpal fractures, the time efficiency and decreased implant costs of using 2-2.0 mm lagged screws may be preferred.

BIOMECHANICS OF THE NORMAL AND DISEASED METACARPOPHALANGEAL JOINT: IMPLICATIONS ON THE DESIGN OF JOINT REPLACEMENT IMPLANTS

The metacarpophalangeal (MCP) joint is crucial for hand function, but is frequently affected by arthritis, leading to pain and disability. This paper reviews the biomechanics of the normal and diseased joint in order to help consider the design of improved MCP joint replacement implants. The normal MCP joint enables a large range of motion in flexion/extension and abduction/adduction as well as a few degrees of rotation. A normal joint typically allows 90° flexion, with a grip strength of up to 672 N. The diseased joint has a reduced range of motion (typically 30° flexion) and reduced hand strength compared to the normal joint. Current MCP joint replacement implants generally try to recreate the range of motion of the normal joint; however, many designs are prone to fracture, as they are unable to withstand the conditions of the diseased joint. It may be beneficial for future implant designs to provide just a functional range of motion. Future designs of MCP joint replacement implants need to be more durable and last longer. Careful consideration of the diseased joint, rather than the normal joint, may help to better define the requirements for such implants.

Preliminary results of nonconstrained pyrolytic carbon arthroplasty for metacarpophalangeal joint arthritis

PURPOSE

To review early outcomes of arthritic metacarpophalangeal (MCP) joints treated with nonconstrained pyrolytic carbon implants to evaluate efficacy, clinical outcomes, and durability.

METHODS

One hundred forty-two consecutive arthroplasties (61 patients) were retrospectively reviewed. Diagnoses included osteoarthritis (OA), traumatic arthritis, and inflammatory arthritis. One hundred thirty were primary joint replacements, and 12 were prior-silicone revisions. The average patient age was 55 years (range, 21-77 years); 36 patients were women and 25 were men. Average follow-up period was 17 months (range, 3-42 months), and 43 patients were followed up for a minimum of 1 year.

RESULTS

For OA patients, according to the analog pain scale used, pain decreased from 73.0 to 8.5 of 100, functionality increased from 20.1 to 86.6 of 100, and appearance improved from 62.7 to 93.6 of 100. The rheumatoid arthritis (RA) group showed decreased pain from 43.1 to 8.9 of 100, functional improvement from 26.7 to 83.3 of 100, and increased appearance from 25.2 to 77.1 of 100. At 1 year, satisfaction was greater than 90% for both groups. Arc of motion for OA patients improved from 44 degrees to 58 degrees . Oppositional pinch increased 126%, and grip strength improved 40%. Rheumatoid arthritis patients increased their MCP joint motion arc from 32 degrees to 45 degrees . Oppositional pinch increased 89%, but grip strength decreased. Radiographs at 1 year demonstrated stable prostheses in all of the OA joints. The RA group overall demonstrated evidence of axial subsidence (10.5% of joints) and periprosthetic erosions (16.4% of joints). In RA joints with greater than 1-year follow-up period, 55.0% had axial subsidence, 95.0% had an increased radiolucent seam, and 45.0% had periprosthetic erosions. The overall implant survivorship is 141 joints to date. The overall minor complication rate was 6%, and major complication rate was 9%.

CONCLUSIONS

Preliminary results suggest that pyrolytic carbon MCP joint arthroplasty provides good pain relief, patient satisfaction, and functional improvement in managing OA and select cases of RA. Longer follow-up evaluation will help validate these promising early results.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic IV.

FEA analysis of silicone MCP implant

This paper discusses finite element study of silicone rubber prosthesis for the metacarpophalangeal joint of the hand. Based on the experimental data, a material model which incorporates test data available for different stress states was chosen and calibrated. Finite element models for three commercially available silicone joint prosthesis were developed. All models incorporated the same material model and allowed for large deformations. These models were validated against the experimental data and analyzed under demanding loading conditions. Results such as highly non-linear material behavior, dependence on the loading history and large deformations near wrinkle formation in the hinge area of the joint clearly show the necessity and importance of using multi-stress- state non-linear material models and accounting for large deformations.

New-generation implant arthroplasties of the finger joints

Prosthetic replacement in the hand must address such unique challenges as preservation of the collateral ligaments, tendon balancing, and stability. Some recently developed implant arthroplasties of the metacarpophalangeal and proximal interphalangeal joints have anatomically designed articular components; others have non-cemented, press-fit, carefully contoured intramedullary stems. The rationale behind developing the unlinked or semiconstrained prosthesis with anatomic geometry is that it would create balanced forces across the joint. Low-profile, anatomically designed implants limit the amount of bone removed and preserve the integrity of the collateral ligaments. A metacarpophalangeal joint implant with an elliptical metacarpal head and a nonfixed center of rotation can enhance stability in flexion through greater articular contact. A proximal interphalangeal joint implant that preserves the collateral ligaments also can achieve improved stability. Component loosening is not an early complication with these recent designs, and arc of motion is satisfactory.

Radiographic assessment of the relative lengths of the bones of the fingers of the human hand

The study assessed whether a relationship existed between the lengths of the phalanges of the fingers of the hand. The centres of rotation of the joints in each finger were determined by dissection of cadaveric hands. Using these data, the distances between the joint centres was determined on anteroposterior hand X-rays taken for clinical purposes. The study has shown that, for all the fingers, there is a ratio of 1 for the distance between the metacarpophalangeal and proximal interphalangeal joint and the distance between the proximal interphalangeal joint and the finger tip. The ratio for the distances between the interphalangeal joints and the distal joint and the tip approximates to 1.3 for the index, middle and ring fingers and to 1.0 for the little finger. No evidence was found to support Littler's hypothesis that the interarticular distances of the finger follow the Fibonacci sequence.

Design considerations for a wrist implant

Rheumatoid arthritis and other forms of inflammatory arthritides commonly affect the wrist leading to pain, deformity and a reduced quality of life for the patient. Joint arthroplasty is an attractive solution for improving function while relieving pain, but unfortunately, current designs of wrist arthroplasties have not met with great success. This review paper describes the anatomy and biomechanics of the normal wrist, and reviews the current and past designs of artificial wrist joints. The design considerations for a successful wrist implant are discussed, and it is concluded that future generations of wrist implants should not attempt to recreate the natural wrist, but permit a limited functional range of motion. Different materials and methods of fixation of artificial wrist joints should also be considered to improve implant durability.