Trevor C. Hearn

Insertional torque and pull-out strengths of conical and cylindrical pedicle screws in cadaveric bone.

Study Design. Insertion torque and pull‐out strengths of conical and cylindrical pedicle screws were compared in human cadaveric vertebral bodies. Objectives. To compare the performance of the conical design with the cylindrical design, and to determine whether insertional torque correlates with pull‐out strength. Summary of Background Data. A tapered pedicle screw design may lessen the likelihood of implant failure. Its effect on thread purchase is not known. Previous studies of cylindrical designs on the relation between insertion torque and pull‐out strength have been conducted in bovine and synthetic bone. Methods. Seventy‐eight pedicles were assigned randomly to one of the following pedicle screw: Texas Scottish Rite Hospital (Sofamor‐Danek, Memphis, TN), Steffee VSP (Acromed, Cleveland, OH), Diapason (Dimso, Paris, France), AO Schanz (Synthes, Paoli, PA), or Synthes USS (Synthes, Paoli, PA). Pedicle screws were inserted with a torque screwdriver. Each screw was extracted axially from the pedicle at a rate of 1.0 mm/sec until failure using an MTS machine (Bionix 858, Minneapolis, MN). Force data were recorded. Results. The conical design had the highest insertion torque. There were no significant differences in pull‐out between any of the screw types. Correlation between insertional torque and pull‐out strength was statistically significant only with the Texas Scottish Rite Hospital and Steffee VSP in L4 and AO Schanz in L5. Conclusions. A conical screw profile increases insertion torque, although insertional torque is not a reliable predictor of pull‐out strength in cadaveric bone. Screw profile (with similar dimensions) has little effect on straight axial pull‐out strengths in cadaveric bone.

The effect of hydration on the stiffness of intervertebral discs in an ovine model.

OBJECTIVE To determine the hydration-over-time behaviour of ovine intervertebral discs and intact joints in a saline bath at body temperature and the effect this has on their stiffness compared to air at ambient temperature. DESIGN The hydration-over-time behaviour and stiffness of the ovine functional spinal unit and disc were quantified. BACKGROUND The fluid content of an intervertebral disc is not constant but varies with external load and load history. The stiffness of ovine functional spinal units in a hydrated environment and how this compares to testing in air have not been quantified. METHODS Intervertebral discs and functional spinal units were weighed and soaked in a saline water bath at 37 degrees C and reweighed each hour for 6 h. They were then allowed to stand in air at room temperature while the time to return to initial weight was recorded. Functional spinal units were randomly assigned to two groups. Axial compression, flexion, extension, lateral bending and axial torsion tests were performed on both the intact functional spinal unit and isolated disc. Group 1 was tested in air then in a saline water bath at 37 degrees C with the testing order reversed for Group 2. RESULTS Hydration of the disc reached a plateau after an average 3-4 h of soaking with the largest increase seen in the first hour. Four hours, standing in air at room temperature, was required to return specimens to their initial weight. The functional spinal unit stiffness was significantly lower for those specimens tested in the bath compared to air. CONCLUSIONS Ovine intervertebral discs show similar hydration-over-time behaviour when compared to human discs. Stiffnesses in different modes of loading were significantly different when tested in a hydrated environment compared with the standard method of testing in air. RELEVANCE It has been shown that there are biomechanical and biochemical similarities between sheep and human intervertebral discs. Despite these similarities, no studies have looked at how ovine intervertebral discs behave over time in a hydrated environment. In humans, hydration levels are an important aspect of intervertebral disc degeneration. There is also a relationship between decreased hydration levels and increased stiffness. This study demonstrates the similarities between human and ovine hydration-over-time behaviour. The importance of intervertebral disc hydration and its effects on stiffness under different modes of loading were also demonstrated and have not been previously shown using the ovine model. In this context, the results from this study provide further support for the use of the ovine model.

Mechanical and pathologic consequences of induced concentric anular tears in an ovine model

Study Design. Relations between induced concentric tears in the sheep disc and the mechanics of the intervertebral joint and vertebral body bone were analyzed. Objective. To examine the effect of concentric disc tears on the mechanics of the spine. Summary of Background Data. Degeneration of the intervertebral disc results in changes to the mechanics and morphology of the spine, but the effect of concentric disc tears is unknown. Methods. In this study, 48 merino wethers were subjected to surgery, and discs were randomly selected for either a needlestick injury or induction of a concentric tear in the anterior and left anterolateral anulus. Sheep were randomly assigned to groups for killing at 0, 1, 3, 6, 12, and 18 months. From each sheep, two spine segments were mechanically tested: one with a needlestick injury and one with a concentric tear. Macroscopic disc morphology was assessed by three axial slices of the disc. Sagittal bone slices were taken from cranial and caudal vertebral bodies for histologic analysis. Results. Induced concentric tears decrease the stiffness of intact spine segments in left bending and the disc alone in flexion. In all other mechanical tests, the needlestick injury had the same effect as the induced concentric tear. In the isolated disc, the disc stiffness at 6 months was increased for right bending, as compared with the response at 1 month. This was associated with increased anterior lamellar thickening and increased vertebral body bone volume fraction. Conclusions. Concentric tears and needlestick injury in the anterior anulus lead to mechanical changes in the disc and both anular lamellar thickness and vertebral body bone volume fraction. A needlestick injury through the anulus parallel to the lamellae produces progressive damage.

Effect of screw torque level on cortical bone pullout strength

Objectives: The objectives of this study were 2-fold: (1) to perform detailed analysis of cortical screw tightening stiffness during automated insertion, and (2) to determine the effect of 3 torque levels on the holding strength of the bone surrounding the screw threads as assessed by screw pullout. Methods: Ten pairs of ovine tibiae were used with 3 test sites spaced 20 mm apart centered along the shaft. One side of each pair was used for measuring ultimate failure torque (Tmax). These Tmax and bone-density values were used to predict Tmax at contralateral tibia sites. Screws were inserted and tightened to 50%, 70%, and 90% of predicted Tmax at the contralateral sites to encompass the average clinical level of torque (86% Tmax). Pullout tests were performed and maximum force values were normalized by cortical thickness. Results: Torque to failure tests indicated tightening to 86% Tmax occurs after yield and leads to an average 51% loss in stiffness. Normalized pullout strength for screws tightened to 50% Tmax, 70% Tmax, and 90% Tmax were 2525 ± 244, 2707 ± 280, and 2344 ± 346 N, respectively, with a significant difference between 70% Tmax and 90% Tmax groups (P < 0.05). Conclusions: Within the limitations of our study involving the testing of 1 type of screw purchase in ovine tibiae, results demonstrate that clinical levels of lag screw tightening (86% Tmax) are past the yield point of bone. Tightening to these high torque levels can cause damage leading to compromised holding strength. Further research is still required to establish the appropriate level of torque required for achieving optimal fracture fixation and healing.

Bone plate fixation: an evaluation of interface contact area and force of the dynamic compression plate (DCP) and the limited contact-dynamic compression plate (LC-DCP) applied to cadaveric bone.

OBJECTIVES To determine the effects of bone morphology on the interfacial mechanics of bone plate fixation. DESIGN A comparison of two bone plate designs using measures of contact area and force in three conditions of bone type. INTERVENTION The influence of bone plate design, bone type, and implant location on interfacial characteristics of contact area and average force were evaluated using pressure-sensitive film in cadaveric bone. Data were quantitated using computer-assisted image analysis. MAIN OUTCOME MEASUREMENTS Means of the two dependent variables in each of the three conditions were compared using analysis of variance. RESULTS In human femora and equine third metacarpi, there were no apparent differences in interface contact area attributed to bone plate design, although there were significant differences for interface contact force. In humeri, significant differences in both interface contact area and force were found between bone plate designs. CONCLUSIONS Results show that bone surface topography and implant location are major determinants of the interfacial elements of contact area and average force between a plate and the bone to which it is applied.

Number, incidence and projections of distal forearm fractures admitted to hospital in Australia

BACKGROUND This study documents the number, incidence, and relative risk (RR) of distal forearm fractures requiring admission to hospital in Australia in 1997 and estimates of the projected numbers for 2021. METHODS Distal forearm fracture numbers from 1993 to 1998 were obtained. The incidence and RR were calculated by gender and 5-year age groups for 1997. Projected numbers for 2021 were estimated using population projection data. RESULTS In 1997, fracture numbers were 12357 for male patients and 19319 for female patients. The incidence was 152 per 100000 for male patients and 157 per 100000 for female patients. The highest incidence and RR was in the 10- to 14-year age group for male patients and in the 85 years and over age group for female patients. If fracture incidence remains constant (on the basis of predicted population changes in Australia), by 2021, it is estimated that fracture numbers for people 50 years of age and over will increase by 81%, compared with 11% for people under 50. In the older age group, the number for women will be 4.7 times higher than for men. CONCLUSION This study quantifies the potential disproportionate increase in distal forearm fractures requiring admission in Australia, particularly for female patients, because of an aging population.

Cement Penetration and Stiffness of the Cement-Bone Composite in the Proximal Tibia in a Porcine Model

Purpose. To assess the stiffness of the cement bone composite and the depth and uniformity of cement penetration into the surface of the tibial component during total knee reconstruction in a porcine model. Methods. The effectiveness of 3 protocols were compared: 2 commonly used cementing techniques—finger-packing of cement on the cut surface followed by impaction, and coating of the undersurface of the prosthesis with cement followed by impaction—and a new method using a tibial cement-pressurising device. Cement penetration was measured by computed tomography; stiffness was determined by hydraulic penetration testing. Results. Cement penetration at a depth of 1 mm was significantly greater following coating the undersurface of the prosthesis than following finger-packing (p=0.008). There was no significant difference at deeper levels or between the tibial-pressurising device group and either of the 2 other groups at any level (p>0.3 in all cases). Differences in surface stiffness by tibial plateau region were found in tibiae that had been cemented using finger-packing and in those that had had their undersurface coated, but not in tibiae that had been cemented using the tibial-pressurising device. Conclusion. The tibial cement-pressurising device eliminated regional differences in stiffness seen with other cementing methods. Elimination of these differences by using this device should reduce micromotion and the incidence of aseptic loosening of tibial base plates in total knee arthroplasty.

Comparison of Torsional Strengths of Bioabsorbable Screws for Anterior Cruciate Ligament Reconstruction

The goals of this study were to evaluate torsional strength and modes of failure in commercially available bioabsorbable interference screws and to test the effect of screw diameter on torsional strength when screws become jammed during insertion. We tested the Arthrex, BioScrew, Endo-Fix, Phantom, and Sysorb screws, all 20 mm in length. Four major modes of failure were encountered. Analysis of variance revealed that both screw type and diameter had a significant effect on failure torque. The Endo-Fix 7-mm screw had the lowest failure torque (1.07 ± 0.18 N·m) and the Sysorb 8-mm screw had the highest (5.23 ± 0.24 N·m). The Sysorb was significantly stronger than all the other screws. The failure torques were within the range that has been reported for manual screw insertion. We concluded that technical factors, which can affect insertion torque, assume particular importance with the use of bioabsorbable interference screws.

Biomechanical analysis of the Medoff sliding plate

BACKGROUND A new device for fixation of hip fractures the Medoff plate was biomechanically tested and compared with the Ambi plate and Gamma nail. METHODS Six pairs of human cadaveric femurs were used. The characteristics evaluated were structural stiffness, strain distribution, and modes of failure. Results were compared with the Ambi plate and Gamma nail biomechanical studies from a previous study that used the same methods. RESULTS The Medoff plate was stiffer than the other two implants for intertrochanteric fractures, and for segmental subtrochanteric fracture. The Medoff plate was more load sharing than the other implants in these fracture configurations. The mean load to failure was lower than for the Ambi plate or the Gamma nail. CONCLUSION The Medoff plate is a better load-sharing device than the Ambi (DHS) or Gamma nail systems. The main concern is its structural weakness. The implant failed at loads 50% less than other devices. The greatest risk of implant failure is with unstable subtrochanteric fractures.

A CAD System for Long-Bone Segmentation and Fracture Detection

Medical imaging has advanced at a tremendous rate since x-rays were discovered in 1895. Today, x-ray machines produce extremely high-quality images for radiologists to interpret. However, the methods of interpretation have only recently begun to be augmented by advances in computer technology. Computer aided diagnosis (CAD) systems that guide healthcare professionals in making the correct diagnosis are slowly becoming more prevalent throughout the medical field. Detection of long-bone fractures is an important orthopaedic and radiologic problem, and it is proposed that a novel CAD system could help reduce the number of fractures missed during x-ray diagnosis. A number of image processing software algorithms useful for assisting the fracture detection process are described, and their accuracy evaluated on a database of fracture images from trauma patients. Incorporating these methods will further expand the capabilities of todays CAD systems, and result in more accurate diagnosis of fractures and a reduction of the fracture miss rate.