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Biomedical subjects

H K Dunn

Publications and source records attributed to H K Dunn.

18 recordsLinked to original sources
PubMed1991

A new method for assessing relative dynamic motion of vertebral bodies during cyclic loading in vitro.

A new experimental technique for measuring generalized three-dimensional motion of vertebral bodies during cyclic loading in vitro is presented. The system consists of an orthogonal array of three lasers mounted rigidly to one vertebra, and a set of three mutually orthogonal charge-coupled devices mounted rigidly to an adjacent vertebra. Each laser strikes a corresponding charge-coupled device screen. The mathematical model of the system is reduced to a linear set of equations with consequent matrix algebra allowing fast real-time data reduction during cyclic movements of the spine. The range and accuracy of the system is well suited for studying thoracolumbar motion segments. Distinct advantages of the system include miniaturization of the components, the elimination of the need for mechanical linkages between the bodies, and a high degree of accuracy which is not dependent on viewing volume as found in photogrammetric systems. More generally, the spectrum of potential applications of systems of this type to the real-time measurement of the relative motion of two bodies is extremely broad.

Equipment Design↗
PubMed1989

Initial effect of collarless stem stiffness on femoral bone strain.

Stress shielding resulting from a stiffness mismatch between bone and femoral prosthesis stems (leading to bone resorption in the proximal femur) is believed to contribute to failure in total hip arthroplasty. In this study, strains were measured under compressive femoral head loads both in the intact femur and after implanting first a collarless steel stem and then a geometrically identical fiber-reinforced polymer composite stem 64% less stiff. Decreasing stem stiffness would be expected increase load transfer from the stem to the proximal medial femur, decreasing the degree of stress shielding. The authors found that proximal medial bone strains were significantly lower with either the steel or composite stem implanted than in the intact case. However, there were no significant differences in strain patterns between the steel and composite stem cases. This apparent insensitivity to prosthesis stiffness may result from factors related to implant geometry and fit.

Adult↗
PubMed1986

Anterior spine stabilization and decompression for thoracolumbar injuries.

In a series of patients with thoracolumbar spine injuries, anterior spinal canal decompression resulted in better neurologic recovery than did previously reported posterior instrumentation or nonoperative treatment. The technique allows stabilization over a much shorter segment of the spine than posterior instrumentation and therefore is indicated for fractures at L2 and below and in all patients with burst fractures and neurologic compromise.

Adolescent↗
PubMed1985

Computerized tomographic determination of vertebral density after total hip arthroplasty.

Computerized tomography (CT) provides the capability to determine vertebral bone density with a high degree of accuracy and precision, detecting density changes as small as 5% in serial testing. In this pilot study, CT was used to determine the effects of total hip arthroplasty and its consequent alterations in physical activity levels on the vertebral bone density of 13 postoperative patients. Vertebral bone density was also evaluated in six preoperative patients with degenerative joint disease of the hip and five control patients. The preoperative group and controls were then compared with the postoperative patients, who were divided into two groups--those who had achieved good activity levels and those whose levels of activity remained poor. CT measurement of bone density in the central bodies of T12, L1, and L2 showed that the preoperative group was similar to controls. This suggests that patients accept total hip arthroplasty rather than endure significant activity limitations for extended periods of time. When activity level was restored by arthroplasty, vertebral bone density was preserved. If the patient continued at a poor activity level postoperation, significant loss of bone density occurred secondary to disuse.

Aged↗
PubMed1985

Strength of stainless steel surgical wire in various fixation modes.

Although wire is a commonly used fixation device in orthopedics, little is known about the effects of fastening methods and configurations on the strength of wiring systems. Three sizes of stainless steel surgical wire were tested in noncyclic tensile loading. Load failure levels increased with enlarging wire diameter. Fastening twists were stronger than knots or the ASIF bend technique. The failure mode for twists was untwisting. More than two twists gave no additional strength to the system. Two commercial wire tightening devices gave stronger and more reproducible twists than did ordinary pliers. The addition of a tension equalizing loop opposite the fastening loop weakened the wire system by 10%-15%. The combination of two single wire loops was stronger than a continuous double loop of wire. The results from this study are intended to help the orthopedic surgeon optimize the use of wire.

Fracture Fixation, Internal↗
PubMed1984

A new method for continuous intraoperative measurement of Harrington rod loading patterns.

A new method has been developed for continuous intra-operative monitoring of bone and implant loading patterns in the operative correction of scoliosis using the Harrington distraction rod. The method incorporates a strain gauge instrumented, distal Harrington hook and similarly equipped Harrington operating instruments (outrigger and distractor). These latter instruments are used to calibrate the hook's response to bone loads under existing operative conditions. Alternative methods of monitoring loading patterns were explored and rejected because they were noncontinuous, more difficult to use, altered the mechanics of the system, or required more expensive components. The method established is being used to avoid bone rupture or rod damage while gaining maximum spinal correction and to determine the effect of auxiliary implant components on loading patterns.

Biomedical Engineering↗
PubMed1984

Anterior stabilization of thoracolumbar injuries.

A new implant system has been designed to rigidly fix the spine anteriorly at a single level above and below the injury site for treatment of severe thoracolumbar fractures. The primary system consists of two rods rigidly linked by vertebral body bridges. These bridges are secured to each vertebra at two points: one posterolaterally on the body with a vertebral body screw and the other more anterolaterally with a vertebral body staple. This implant system has been used in 48 consecutive thoracolumbar injuries. There have been three late failures to attain spine fusion, but in no patient has there been loss of vertebral column alignment or spinal canal decompression. There have been no major catastrophes with the system. There have been one deep- and two superficial-wound infections, all of which have been successfully treated and resolved.

Adolescent↗
PubMed1979

Degradation resistance of some candidate composite biomaterials.

The degradation resistance of matrix, fiber and composite systems which we have been studying as candidate orthopedic materials has been examined in two appropriate environments. Both resistance to steam sterilization in an autoclave environment and resistance to a simulated physiologic solution have been studied. In the autoclave study, samples were placed in a pressure cooker at 123 degrees C for differing amounts of time and tested for retention of mechanical properties. Results indicate that most of the materials tested could be autoclaved several times, as long as autoclave times did not exceed 1 hr. Longer autoclave times result in an accelerated degradation and loss of strength of all materials except the polypropylene. Polysulfone degrades after even the shortest autoclave duration. Resistance to the simulated physiologic environment was tested by measured retention of mechanical properties after immersion times in pseudo-extracellular fluid (PECF) at 37 degrees C for as long as three years. None of the materials showed any significant changes in properties after immersion in the PECF.

Amides↗
PubMed1977

Fixation of Dwyer screws for the treatment of scoliosis. A postmortem study.

The strength of fixation of Dwyer screws in seven or eight vertebral bodies in each of six fresh osteoporotic cadavera was measured under the following conditions: with fixation with only the screw; with the staple-screw combination; with each of these modes of fixation combined with polymethylmethacrylate placed in the hole prior to insertion of the screw; and with the screw reinserted after failure and filling the hole with methacrylate. The strength of fixation of the screw-staple combination was increased almost twofold by the methacrylate, and after failure followed by reinsertion of the screw embedding it in methacrylate, strong fixation was restored.

Bone Cements↗
PubMed1976

Radiology of knee joint replacement.

As total knee replacement becomes more successful due to advances in surgery and bioengineering, a greater number of patients will be offered this procedure as a viable alternative to arthrodesis. Radiologic consultation will be sought for evaluation of proper alignment as well as postoperative complications such as loosening, instability, dislocation, limitation of range of motion, fracture, and infection. This evaluation may be accomplished by plain films and arthrography. Radiographic aspects of these features are discussed and illustrated.

Biomechanical Phenomena↗
PubMed1976

Ski pole thumb injury.

Disabling injuries can occur in the area of the MP joint of the thumb when a skier falls against the planted ski pole while holding the ski strap in the usual fashion. This is presently the most common ski injury of the upper extremity. It is important to determine if instability has resulted, because there may be disruption of the ulnar aspect of joint capsule as well as of the extensoradductor hood mechanism. If this has occurred, it is immediately to prevent the development of the chronic "gamekeeper's thumb."

Athletic Injuries↗
PubMed1976

Total hip reconstruction in chronically dislocated hips.

The pathological anatomy of chronically dislocated hips makes reconstruction for more difficult in them than in most cases. The acetabular component must be seated at the site of the original triradiate cartilage and the femur must be shortened four or more centimeters to prevent excessive limb lengthening. This means that the femoral component must be seated in the smallest, strightest portion of the intramedullary canal. A specially designed prosthesis is often needed. Twenty-two hips were operated on in this series, and all patients had relief of pain and improvement of gait. One major complication occured: a sciatic-nerve palsy due to overlengthening of the femur and improper postoperative positioning.

Acetabulum↗
PubMed1975

A single unit digital flexor tendon prosthesis.

Historically, the surgical repair of serious injuries to the flexor tendons of the hand have met with less than satisfactory clinical results. In an attempt to solve this problem, studies have been made on the design and testing of a Dacron polyester tendon prosthesis for complete replacement of the digital flexor tendon. The prosthesis attaches proximally at the anatomical musculotendon junction via collagen ingrowth into a fabric structure and in similar manner distally via bone ingrowth at the anatomical insertion site. These fabrics are continuous with the tendon body of the prosthesis consisting of an inextensible braided cord which is silicone rubber coated to prevent tissue adherence in the glide zone of the tendon bed. An added benefit of the prosthesis is a simple means of length adjustment which can easily be carried out on the operating table. Studies have been performed in both dogs and chickens which indicate that sufficient tissue ingrowth occurs at both the distal and proximal anastomoses for retention of full load bearing capability in these animals and that postoperative adhesions are negligible, if present at all. Full prostheses have been implanted in dogs for periods of over 1 year and chickens for 2 months. The chicken is the preferred animal model as a tendency for extensive scar tissue generation was shown in the dog. Both in vivo mechanical testing and histological characterization have been made on sacrificed animals which have received the full prostheses.

Animals↗