N.K Fowler

Method of determination of three dimensional index finger moment arms and tendon lines of action using high resolution MRI scans

High-resolution MRI scans, in conjunction with CAD software, were used to determine the three-dimensional moment arms and force vector direction cosines for 11 structures passing the interphalangeal and metacarpophalangeal joints of the index finger. The results are presented for five different angles of joint flexion for a single subject. The moment arm data obtained differ from previous studies, where results have been derived from tendon excursion techniques or geometrical models. These dissimilarities have been accounted for by the differences in experimental techniques.

Interphalangeal joint and tendon forces: normal model and biomechanical consequences of surgical reconstruction

Soft tissue reconstructive surgery for rheumatoid-related proximal interphalangeal joint deformities frequently fails to produce the long-term predicted results. Detailed information on the biomechanics of this joint, under both normal and pathological conditions, is required to assess the efficacy of such surgical intervention. A biomechanical model of the proximal interphalangeal joint has been developed to investigate tendon and joint loading during real life three-dimensional activities. Based on a rigid body mechanics approach, the model uses high resolution MRI scans to obtain anatomical tendon and bone geometries in conjunction with three-dimensional kinematic and loading data. The model incorporates an optimisation routine which minimises overall maximum tendon stress in the eight individual elements considered. Radial and ulnar joint force components are included at the proximal interphalangeal joint level. Two simulated pathological versions of the mathematical model are developed to accommodate the altered anatomic relationships after tendon reconstructive surgery. Joint forces of up to 450N and common usage of the extensor mechanism during normal pinching and grasping activities are predicted. The ulnar lateral bands of the extensor tendon are generally loaded to a greater extent than the radial bands. Extensor tendon and joint forces in the simulated pathological models are significantly higher than those in the normal model. Combined with the poor tendon quality of rheumatoid arthritis patients generally, these amplified internal forces may lead to further joint deformation.

A force transducer to measure individual finger loads during activities of daily living.

A new six-degree-of-freedom force transducer has been manufactured, with the sensitivity to measure forces in the range +/-100 N and moments of up to +/-5 Nm. The transducer incorporates two mechanical components: shear forces and bending moments are measured via a strain-gauged tubular section whilst axial forces are transmitted to a cantilevered load cell. Both components are instrumented with 350 ohms strain gauge full bridge circuits and are temperature compensated. After calibration, measurement errors are less than +/-0.3 N for direct forces and +/-0.03 Nm for applied moments. In order to measure sub-maximal finger loads during activities of daily living, the transducer has been incorporated into several housings representing objects in domestic use: a jar, a tap, a key in a lock and a jug kettle.

A biomechanical analysis of the rheumatoid index finger after joint arthroplasty

OBJECTIVE To determine the mechanisms responsible for the recurrence of ulnar drift after metacarpophalangeal joint arthroplasty in the rheumatoid hand. DESIGN A three-dimensional biomechanical model of the index finger joints was used to predict the implant loads during several activities of daily living. BACKGROUND Post-operative clinical evaluation of Sutter metacarpophalangeal prostheses shows a high incidence of fracture and recurrent deformity. METHODS A six-component force transducer in conjunction with a six-camera motion analysis system were used to obtain kinematic and external loading data from eight patients with rheumatoid arthritis during several simulated activities. These data were used as input into a three-dimensional biomechanical model of the implant and interphalangeal joints of the index finger. Tendon lines of action and moment arms were obtained using a series of MRI scans and CAD modelling techniques. RESULTS Implant forces were oriented in a radial and dorsal direction to resist the ulnarpalmarly pull of tendons associated with the metacarpophalangeal joint. CONCLUSIONS The recurrence of ulnar drift is attributable to fatigue failure of the prostheses. After fracture the implant is unable to support the repetitive loading patterns experienced during activities of daily living. RELEVANCE Understanding the mechanisms responsible for the recurrence of ulnar drift and implant failure is a step towards improving the prosthesis design, surgical procedures and ultimately the patients prognosis.

Reciprocating gait prosthesis for the bilateral hip disarticulation amputee

Abstract This paper describes the design, manufacture and use of a new prosthesis which produced reciprocating gait for a bilateral hip disarticulation amputee. A special mechanism used the hip extension moment during weight bearing to drive the contralateral limb through the swing phase. The user rapidly attained efficient and safe reciprocating gait, together with simple donning and doffing. The success of this project has major implications for hip disarticulation amputees in terms of profound improvements of their independence and self-esteem.

The development of a modular humeral prosthesis locking mechanism

Abstract A new modular humeral prosthesis has been developed, incorporating a novel locking mechanism to prevent head-stem disassembly. A series of test protocols was designed to evaluate the integrity of the new locking mechanism and a dedicated test rig was manufactured. Preliminary test results were disappointing and the locking mechanism failed at lower applied loads than predicted. Microscopy studies were conducted to establish the cause of this poor performance and with the aid of statistical analysis, these studies assisted in the development process. Development of the design was directed at the control of the position and angle of contact between bearing surfaces of the locking mechanism. The optimized design was tested under the maximum anticipated loading conditions and performed satisfactorily.