Performance of digital filtering methods in orthotic and prosthetic CAD/CAM.

This report characterizes the performance of three digital filters, when applied to residual limb shape maps. The three filters were an averaging filter, a uniform window Fourier filter, and a Hamming window Fourier filter. The frequency responses of the three filters were calculated from theory, and experimentally observed. Experimental observations consisted of responses on single-frequency lobed shapes, and on residual limb shapes. Seven trans-tibial limb molds were digitized, three times each. Each resulting shape was then passed through each of the three filters. The before and after shapes were then compared. A Hamming window filter (low-pass frequency of 10 cycles per revolution, 24 coefficients, Hamming number of 0.25) achieved the best performance based on maintained amount of residual limb frequencies, and on visual observation.

A method of residual limb stiffness distribution measurement.

A method of recording a residual limb indentation stiffness map was developed for possible use as an aid in calculating prosthetic socket rectifications. The method was tested to determine the level of repeatability attainable. A hand-held, pencil-like device was used, with an air-driven piston that indented the tissue 10 times per second. The indentor tip contained an electromagnetic digitizer element that sensed position and orientation 120 times per second. The examiner moved the device around the limb; sampling was variable in density, and typically concentrated on critical areas. An interactive visual display of sampled data quality was used to guide sampling. The indentation maps typically contained approximately 4,000 locations, in a cylindrical coordinate system, with sampling locations spaced every 3.2 mm vertically, and every 0.087 radians tangentially. The behavior of the system was characterized using six test subjects on whom recorded indentations ranged from 1.5 to 21 mm. The largest range of indentations (i.e., worst disagreement) recorded at a single location was 5.4 mm. The average standard deviation on repeated measurement ranged from 7 to 15%, and averaged 0.67 mm in absolute terms. Many of the structurally significant anatomical features of the limbs were visible, including the patella and patellar tendon, fibular head, shin, biceps femoris tendon, semitendinosus, and popliteal area.

A survey of function in children with lower limb deficiencies.

Function and prosthesis technical problems were surveyed in 258 experienced paediatric lower-limb prosthesis wearers. The two-part survey form consisted of the modified Prosthesis Evaluation Scale and the core module of the American Academy of Orthopaedic Surgeons/Council of Musculoskeletal Specialty Societies (AAOS/COMSS) Lower Limb Outcomes instrument. Eighty-eight percent (88%) of these paediatric subjects were able to wear their prosthesis more than 9 hours/day; only 3 subjects (1%) were not able to wear their limb at all. The average distance walked per day was reported to be 5.24 kilometres. Sixteen percent (16%) reported pain as "moderate" or worse. A majority reported not having a problem with perspiration, however, 20% had problems serious enough to limit prosthesis wearing time significantly. The most common reasons for temporary loss of limb use were pain (62 responses) and prosthesis failure (59 responses), followed by tissue breakdown (42 responses) and perspiration (30 responses). In general, the paediatric population achieves full use at a high rate, is much more active than the adult population, and experiences less limb pain.

Indentor tests and finite element modeling of bulk muscular tissue in vivo.

The quasi-static response of bulk muscular tissue to indentation was measured on the posterior lower legs of living human subjects. No residual limbs were tested; all subjects had intact lower limbs. For loads up to 7.0 N on an 8.0 mm diameter flat-tipped indentor, the response was repeatable without prior 'preconditioning'. The data at any test location exhibited substantial random scatter, but did not trend up or down with repeated cycles. At these limited loads (< 7.0 N), hysteresis was always evident but was always < or = 10% of the maximum reaction force generated. At these limited loads, stress relaxation, in the time period between 5 and 1200 seconds after indentation, was < 10% (> 90% confidence). At higher load levels (> 12.0 N), greater hysteresis and prolonged stress relaxation were observed, accompanied by minor tissue damage. In order to estimate the composite material stiffness of the tissue, the indentations were modeled using a materially and geometrically nonlinear, large-strain finite element formulation. The resulting composite material stiffness was nonlinear, and could be approximated using the Jamus-Green-Simpson strain energy function; typical values for the coefficients were c10 = 0.0026 MPa, c01 = 0.00064 MPa, and c11 = 0.0057 MPa.

Anterior cruciate ligament graft tensioning in full extension.

The purpose of this study was to determine anterior laxity and graft forces in cadaveric knees after anterior cruciate ligament reconstruction in which the graft is tensioned with the knee in full extension. We also analyzed the clinical results from a series of patients who had ligament reconstructions using this technique. We performed anterior cruciate ligament reconstructions on seven fresh cadaveric knees and then measured the anterior laxity, graft set force, and graft tension. We also did a prospective minimum 2-year followup on 57 patients who underwent anterior cruciate ligament reconstruction. The in vitro data showed average anterior laxity of 1.1 mm greater than the intact knee with an 89-N anterior force at 30 degrees of knee flexion. Average graft set force was 68 N, and graft tension in extension was 18 N. In the clinical portion of the study, knee laxity improved from 7.5 to 0.8 mm (side-to-side difference at 89 N). The patients' Lysholm and Tegner scores improved from 65 to 90 and 3.9 to 5.6, respectively. Only one patient had a postoperative contracture. The results of this study suggest that graft tensioning in full extension provides a low incidence of flexion deformity, maintaining excellent functional results and satisfactory biomechanics.

Modelling the mechanics of narrowly contained soft tissues: the effects of specification of Poisson's ratio.

Many soft tissues are considered to be virtually incompressible. A number of recent analyses of the mechanics of these tissues have used Poisson's ratios in the range of 0.45 to 0.49 with little or no documentation, the apparent assumption being that a small change in Poisson's ratio will not significantly affect the results. We demonstrate here that the mechanics of a narrowly contained soft tissue are, instead, strongly sensitive to small changes in compressibility about the incompressible limit. Relevant practical examples include analysis of the mechanics of soft tissues within the sockets of artificial legs, pressure sore problems, and the calculation of strains within the soft tissues of a fracture gap.