Biomedical subjects

Kent N Bachus

Publications and source records attributed to Kent N Bachus.

26 records · Page 2Linked to original sources

PubMed2003

Tibial component fixation with cement: full- versus surface-cementation techniques.

Despite excellent outcomes with cemented tibial components in total knee arthroplasty, it still is debated whether the tibial stem should be cemented and what the optimal tibial stem design should be. Proponents of full cementation of the tibial stem and component state that better short-term and long-term component fixation is achieved when full cementation is used. Advocates for surface cementation contend that sufficient implant stability is achieved without the increased bone loss that occurs at revision and the stress shielding thought to be linked with cemented stems. This biomechanical cadaver study compared initial fixation and cement penetration depth in fully cemented versus surface cemented tibial trays with two different stem geometries (cruciate and I-beam) and compared two stem designs (cruciate and I-beam) fixed with surface cementation. Under an eccentric load, simulating three times body weight for 6000 cycles, there seems to be no difference in the micromotion of either tibial component implanted with surface or full cementation. Additionally, no difference in the average depth of cement penetration was detected between fixation techniques or stem types. The initial fixation stability of the surface cement technique seems correlated to the depth of cement penetration into proximal tibial surface. The current data support other studies which indicate that stability of surface-cemented tibial components may be related to the depth of cement penetration.

Adult↗

PubMed2003

Are uniform regional safety factors an objective of adaptive modeling/remodeling in cortical bone?

It has been hypothesized that a major objective of morphological adaptation in limb-bone diaphyses is the achievement of uniform regional safety factors between discrete cortical locations (e.g. between cranial and caudal cortices at mid-diaphysis). This hypothesis has been tested, and appears to be supported in the diaphyses of ovine and equine radii. The present study more rigorously examined this question using the equine third metacarpal (MC3), which has had functionally generated intracortical strains estimated by a sophisticated finite element model. Mechanical properties of multiple mid-diaphyseal specimens were evaluated in both tension and compression, allowing for testing of habitually tensed or compressed regions in their respective habitual loading mode ("strain-mode-specific" loading). Elastic modulus, and yield and ultimate strength and strain, were correlated with in vivo strain data from a previously published finite element model. Mechanical tests revealed minor variations in elastic modulus, and yield and ultimate strength in both tension and compression loading, while physiological strains varied significantly between the cortices. Contrary to the hypothesis of uniform safety factors, the MC3 has a broad range of tension (caudo-medial, 4.0; cranio-lateral, 37.7) and compression (caudo-medial, 5.7; cranio-lateral, 68.9) safety factors.

Adaptation, Physiological↗

PubMed2003

Cyclic loading of olecranon fracture fixation constructs.

BACKGROUND: Despite the good results that are usually reported after fixation at the sites of olecranon fractures and osteotomies, problems such as loss of fixation, nonunion, and the need for revision surgery are still encountered. Various types of fixation have been recommended, but few have been evaluated with use of clinically relevant cyclic load testing at appropriate levels of stress. The purpose of the present study was to test multiple olecranon fixation techniques under physiologic cyclic loads. METHODS: We studied ten cadaveric elbows with use of cyclic loading that simulated (1) active range of motion and (2) pushing up from a chair. Each specimen underwent fixation of a simulated 50% transverse olecranon fracture with use of intramedullary and cortically fixed tension band constructs (in randomized order) followed by fixation with a 7.3-mm-diameter cancellous screw with and without a tension band. Displacement transducers were placed posteriorly on the tension side and anteriorly near the articular surface. RESULTS: Both configurations involving the 7.3-mm-diameter cancellous screw provided the most stable fixation-nearly five times better than that provided by the Kirschner-wire techniques. Use of the tension band in conjunction with the intramedullary screw improved the stability of fixation. In none of the constructs did the AO tension band result in compression across the osteotomy gap. CONCLUSIONS AND CLINICAL RELEVANCE: The use of a 7.3-mm screw in conjunction with a tension band provided better fixation of simulated displaced transverse fractures than did the use of Kirschner wires in conjunction with a tension band or the use of a screw only. The AO principle of converting posterior tensile forces to articular compressive forces was not demonstrated in this study. We therefore question the validity of the tension band concept in olecranon fracture fixation and recommend passive rather than active range of motion in the immediate postoperative period to limit fracture distraction.

Analysis of Variance↗

PubMed2003

Rotational stability of a modified step-cut for use in intercalary allografts.

BACKGROUND: Intercalary allografts are used for the reconstruction of major skeletal defects. Step-cuts help to provide rotational stability when intramedullary fixation is used. A modified step-cut is proposed to reduce rotation at the interface. This study compares the rotational stability of conventional and modified step-cuts. METHODS: In Phase I, seven pairs of human cadaveric femora were divided into a conventional step-cut group (left femora) and a modified step-cut group (right femora). All femora were cut transversely at the mid-diaphysis. In the conventional group, a 1-cm step-cut was created in the exact midsagittal plane in both the proximal and distal segments. In the modified group, a 1-cm step-cut was created in the parasagittal plane, leaving 2 mm of additional bone on both the proximal and the distal fragment. Phase II was identical except that in the modified step-cut group only 1 mm of additional bone was left. Smooth femoral nails were then placed after standard reaming. Specimens were tested by fixing the proximal segment and applying +/-2 N-m (17.7 in-lb) of torque to the distal segments with ten oscillation cycles. Maximum rotation was measured. The data were analyzed with the paired Student t test. RESULTS: The average rotation in Phase I was 23.3 degrees for the conventional step-cut group and 3.0 degrees for the 2-mm modified step-cut group; the difference was significant (p < 0.001). Four femora sustained an incomplete fracture during nail insertion. The average rotation in Phase II was 20.6 degrees for the conventional step-cut group and 0.5 degrees for the 1-mm modified step-cut group without any fractures; the difference was significant (p < 0.001). CONCLUSIONS: Step-cut modification that leaves more bone in the sagittal plane provides rigid fixation and significantly more stability than the conventional step-cut technique.

Biomechanical Phenomena↗

PubMed2003

Fixation of periprosthetic femur fractures: a biomechanical analysis comparing cortical strut allograft plates and conventional metal plates.

This study compared the stability of periprosthetic femur fractures fixed using cortical allograft struts with a metal plate. Cadaveric specimens were loaded in single-leg stance and stair climbiing to 2250 N. Optimum stability in single-leg stance was achieved with two long struts medially and laterally. No clear advantage was noted in using a second strut in stair climbing. Cables rather than wires were useful in single-leg stance, but not in stair climbing. Allograft cortical struts are a biomechanically sound alternative to metal plates fixed with screws and cables for femur fracture fixation below a well-fixed femoral component.

Biomechanical Phenomena↗

PubMedDate not supplied

Less invasive posterior fixation method following transforaminal lumbar interbody fusion: a biomechanical analysis.

BACKGROUND CONTEXT: Current surgical trends increasingly emphasize the minimization of surgical exposure and tissue morbidity. Previous research questioned the ability of unilateral pedicle screw instrumentation to adequately stabilize posterior fusion constructs. No study to date has addressed the effects of reduced posterior instrumentation mass on interbody construct techniques. Unilateral surgical exposure for transforaminal lumbar interbody fusion (TLIF) allows ipsilateral pedicle screw placement. Theoretically, percutanous contralateral facet screw placement could provide supplemental construct support without additional surgical exposure. PURPOSE: Identify the biomechanical effects of reduced spinal fusion instrumentation mass on interbody construct stability. STUDY DESIGN: An in vitro biomechanical study using human lumbar spines comparing stability of TLIF constructs augmented by: (1) bilateral pedicle screw fixation, (2) unilateral pedicle screw fixation, or (3) a novel unilateral pedicle screw fixation supplemented with contralateral facet screw construct. METHODS: Seven fresh frozen human cadaveric specimens were tested in random construct order in flexion/extension, lateral bending, and axial rotation using +/-5.0 Nm torques and 50 N axial compressive loads. Analysis of torque rotation curves determined construct stability. Using paired statistical methods, comparison of construct stiffness and total range of motion within each specimen were performed using the Wilcoxon signed ranks test with a Holm-Sidák multiple comparison procedure (alpha=0.05). RESULTS: In flexion/extension, lateral bending, and axial rotation, there were no measurable differences in either stiffness or range of motion between the standard bilateral pedicle screw and the novel construct after TLIF. After TLIF, the unilateral pedicle screw construct provided only half of the improvement in stiffness compared with bilateral or novel constructs and allows for significant off-axis rotational motions, which could be detrimental to stability and the promotion for fusion. CONCLUSIONS: All tested TLIF constructs with posterior instrumentation decreased segmental range of motion and increased segmental stiffness. While placing unilateral posterior instrumentation decreases overall implant bulk and dissection, it allows for significantly increased segmental range of motion, less stiffness, and produces off-axis movement. The technique of contralateral facet screw placement provides the surgical advantages of unilateral pedicle screw placement with stability comparable to TLIF with bilateral pedicle screws.

Adult↗

PubMedDate not supplied

Glenohumeral articular contact areas and pressures following labral and osseous injury to the anteroinferior quadrant of the glenoid.

The objective of this study was to determine the effect of progressive labral and bone loss on the articular contact area and pressures across the glenohumeral joint under compressive loads of 220 and 440 N. Eight fresh-frozen cadaver shoulders were used, and contact pressures in 4 quadrants of the glenoid were determined with a Tekscan flexible tactile force sensor. Testing conditions included intact glenoids, glenoids with the anteroinferior labrum removed, and glenoids with 3 sizes of bone defects in the anteroinferior quadrant. By means of Tekscan sensing equipment, the measured contact area over the glenolabral complex was between 49.0% and 61.5% of the calculated surface area for the intact specimens. Loss of the anteroinferior labrum decreased contact area by 7% to 15% compared with the intact specimens, and the mean contact pressure increased by 8% to 20%. With bone loss corresponding to a defect measuring 30% of the diameter in the anteroinferior quadrant, contact area across the entire glenoid decreased a mean of 41% compared with the intact specimens, whereas the mean contact pressure increased nearly 100%. When the anteroinferior quadrant of the glenoid was analyzed separately, loss of the anteroinferior labrum alone resulted in an increase in the mean contact pressure in this quadrant compared with the intact specimens (mean, 53%). Bone loss of 30% of the diameter resulted in mean contact pressures in this quadrant increasing by 300% to 400% compared with the intact specimens, with 2 of 8 specimens becoming grossly unstable. In addition, with 30% diameter bone loss, the mean contact pressure decreased by 26% in the posterosuperior quadrant, indicating a shift in loading of the cadaveric glenoid. Peak pressures followed similar trends, with labral loss alone increasing peak pressures in the anteroinferior quadrant by a mean of 28% of that seen for the intact specimens.

Adult↗

PubMedDate not supplied

Superficial versus deep transfer of the posterior tibialis tendon.

Transfer of the posterior tibialis tendon to the dorsum of the foot is a commonly performed procedure in conditions that weaken ankle dorsiflexors and evertors, resulting in equinovarus foot deformities. When transferring the tendon, surgeons have the choice of routing the tendon deep to the extensor retinaculum or superficial to it. This study compares the biomechanics of these two routing methods. Seven cadaveric lower limbs were tested by applying known forces to the transferred posterior tibialis tendon. Resultant kinematics indicated that passing superficial to the retinaculum resulted in a significantly more efficient motion than transfer deep to the retinaculum.

Adult↗