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.

Automated Bone Screw Tightening to Adaptive Levels of Stripping Torque.

OBJECTIVE: To use relationships between tightening parameters, related to bone quality, to develop an automated system that determines and controls the level of screw tightening. METHODS: An algorithm relating current at head contact (IHC) to current at construct failure (Imax) was developed. The algorithm was used to trigger cessation of screw insertion at a predefined tightening level, in real time, between head contact and maximum current. The ability of the device to stop at the predefined level was assessed. RESULTS: The mean (±SD) current at which screw insertion ceased was calculated to be [51.47 ± 9.75% × (Imax - IHC)] + IHC, with no premature bone failures. CONCLUSIONS: A smart screwdriver was developed that uses the current from the motor driving the screw to predict the current at which the screw will strip the bone threads. The device was implemented and was able to achieve motor shut-off and cease tightening at a predefined threshold, with no premature bone failures.

A new versatile hand dynamometer.

This paper describes the design and testing of a new hand strain-gauge based dynamometer. The design is portable and capable of measuring grip strengths from 12.5 to 800N. The dynamometer is independent of point of application of the grip force. Finite element modelling and experimental testing were used to verify the mechanical system's suitability. The dynamometer was designed for input to a PDA to allow full portability of the device.

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.

The accuracy of three-dimensional reconstructions of the ovine knee: dissectional validation.

A single, complete, ovine knee was imaged using conventional MRI then dissected allowing comprehensive dimensional measurement of the intra-articular structures. A three-dimensional (3-D) computer model of the knee was generated from the MR sections. Thirty-two individual structural measures were recorded from the image output for which there was an available surgical measure for direct comparison. The results of Pearson's correlation testing show a rounded score of 1.00, suggesting an exceptional linear correlation between direct anatomical measurement and the 3-D image output. Further analysis of the data revealed an average error of measurement of 0.2mm across the 32 measures. The findings of this preliminary study suggest that 3-D reconstruction from MR data may be an appropriate, and accurate, means for making dimensional measurements of the bony and soft tissue structures of the ovine knee. It is unlikely that the measurement error would be of any great clinical significance. There is evidence in the literature to suggest that an ovine knee may be considered an acceptable model for substitution for the human knee in diagnostic assessment studies. Therefore, such findings may be considered clinically relevant in the field of human knee assessment.

Predicting cancellous bone failure during screw insertion.

Internal fixation of fractures often requires the tightening of bone screws to stabilise fragments. Inadequate application of torque can leave the fracture unstable, while over-tightening results in the stripping of the thread and loss of fixation. The optimal amount of screw torque is specific to each application and in practice is difficult to attain due to the wide variability in bone properties including bone density. The aim of the research presented in this paper is to investigate the relationships between motor torque and screw compression during powered screw insertion, and to evaluate whether the torque during insertion can be used to predict the ultimate failure torque of the bone. A custom test rig was designed and built for bone screw experiments. By inserting cancellous bone screws into synthetic, ovine and human bone specimens, it was established that variations related to bone density could be automatically detected through the effects of the bone on the rotational characteristics of the screw. The torque measured during screw insertion was found to be directly related to bone density and can be used, on its own, as a good predictor of ultimate failure torque of the bone.

Stem micromotion after femoral impaction grafting using irradiated allograft bone: a time zero in vitro study.

BACKGROUND: A gamma irradiation dose of 15kGy has been shown to adequately sterilise allograft bone, commonly used in femoral impaction bone grafting to treat bone loss at revision hip replacement, without significantly affecting its mechanical properties. The objective of this study was to evaluate whether use of 15kGy irradiated bone affects the initial mechanical stability of the femoral stem prosthesis, as determined by micromotion in a comprehensive testing apparatus, in a clinically relevant time zero in vitro model of revision hip replacement. METHODS: Morselised ovine bone was nonirradiated (control), or irradiated at 15kGy or 60kGy. For each dose, six ovine femurs were implanted with a cemented polished taper stem following femoral impaction bone grafting. Using testing apparatus that reproduces stem loading, stems were cyclically loaded and triaxial micromotion of the stem relative to the bone was measured at the proximal and distal stem regions using non-contact laser transducers and linear variable differential transformers. FINDINGS: There were no significant differences in proximal or distal stem micromotion between groups for all directions (p≤0.80), apart for significantly greater distal stem medial-lateral micromotion in the 60kGy group compared to the 15kGy group (P=0.03), and near-significance in the anterior-posterior direction (P=0.08, power=0.85). INTERPRETATION: Using a clinically relevant model and loading apparatus, irradiation of bone at 15kGy does not affect initial femoral stem stability following femoral impaction bone grafting.

The impact of tensioning device mal-positioning on strand tension during anterior cruciate ligament reconstruction.

BACKGROUND: In order to confer optimal strength and stiffness to the graft in Anterior Cruciate Ligament (ACL) reconstruction, the maintenance of equal strand tension prior to fixation, is desired; positioning of the tensioning device can significantly affect strand tension This study aimed to determine the effect of tensioning device mal-positioning on individual strand tension in simulated cadaveric ACL reconstructions. METHODS: Twenty cadaveric specimens, comprising bovine tibia and tendon harvested from sheep, were used to simulate ACL reconstruction with a looped four-strand tendon graft. A proprietary tensioning device was used to tension the graft during tibial component fixation with graft tension recorded using load cells. The effects of the tensioning device at extreme angles, and in various locking states, was evaluated. RESULTS: Strand tension varied significantly when the tensioning device was held at extreme angles (p < 0.001) or in 'locked' configurations of the tensioning device (p < 0.046). Tendon position also produced significant effects (p < 0.016) on the resultant strand tension. CONCLUSION: An even distribution of tension among individual graft strands is obtained by maintaining the tensioning device in an unlocked state, aligned with the longitudinal axis of the tibial tunnel. If the maintenance of equal strand tension during tibial fixation of grafts is important, close attention must be paid to positioning of the tensioning device in order to optimize the resultant graft tension and, by implication, the strength and stiffness of the graft and ultimately, surgical outcome.

Biomechanical testing of implant free wedge shaped bone block fixation for bone patellar tendon bone anterior cruciate ligament reconstruction in a bovine model.

BACKGROUND: The use of an interference fit wedged bone plug to provide fixation in the tibial tunnel when using bone-patellar tendon-bone autograft for anterior cruciate ligament reconstruction offers many theoretic advantages including the potential to offer a more economical and biological alternative to screw fixation. This technique has not been subjected to biomechanical testing. We hypothesised that a wedged bone plug fixation technique provides equivalent tensile load to failure as titanium interference screw fixation. METHODS: In a controlled laboratory setting, anterior cruciate ligament reconstruction was performed in 36 bovine knees using bone-patella-bone autograft. In 20 knees tibial fixation relied upon a standard cuboid bone block and interference screw. In eight knees a wedge shaped bone block with an 11 mm by 10 mm base without a screw was used. In a further eight knees a similar wedge with a 13 mm by 10 mm base was used. Each specimen used a standard 10 mm tibial tunnel. The reconstructions were tested biomechanically in a physiological environment using an Instron machine to compare ultimate failure loads and modes of failure. RESULTS: Statistical analysis revealed no significant difference between wedge fixation and screw fixation (p = 0.16), or between individual groups (interference screw versus 11 mm versus 13 mm wedge fixation) (P = 0.35). CONCLUSIONS: Tibial tunnel fixation using an impacted wedge shaped bone block in anterior cruciate ligament reconstruction has comparable ultimate tensile strength to titanium interference screw fixation.

Measuring grip strength in normal adults: reference ranges and a comparison of electronic and hydraulic instruments.

PURPOSE: To determine reference ranges for peak, average, and final adult grip strength over 10 seconds by using an electronic dynamometer, and to compare results from hydraulic and electronic dynamometers. METHODS: The hand-grip strengths of 476 healthy adult subjects were tested using the electronic (Grippit; AB Detektor, Goteborg, Sweden) and hydraulic (Jamar; Smith and Nephew, Memphis, TN) dynamometers. RESULTS: Age- and gender-specific reference ranges for the Jamar and Grippit dynamometers are presented. Bland-Altman analysis of the differences between the results obtained using the 2 instruments revealed a bias (mean difference) of 22 N (Jamar - Grippit) and limits of agreement of -86 to 129 N (mean +/- 2 SD), which indicates that grip measurements may vary by up to 215 N between instruments. CONCLUSIONS: The study yielded population reference ranges of peak, average, and final strength over a 10-second grip assessment using an electronic dynamometer. Results from the Grippit and Jamar dynamometers are similar; however, the dynamometers cannot be interchanged. The Grippit provides information about endurance and fatigue of grip over 10 seconds, showing differences between right- and left-dominant adults.

Biceps tenodesis: a biomechanical study of fixation methods.

Rupture of the biceps tendon occurs predominantly in the middle-aged and elderly, being predisposed through bicipital tendinitis and rotator cuff lesions. Surgical repair may be an option for those requiring strength in supination. This study compared the initial fixation strength of keyhole tenodesis (n = 7) and interference screw fixation by use of cadaveric specimens. Two interference screws were evaluated (n = 7 x 2): the round-headed cannulated interference screw (RCI) and a bioresorbable screw (Sysorb). All specimens failed at the fixation site but one. This study found that overall there was a significant effect as a result of study group (keyhole vs Sysorb vs RCI, P =.034). The post hoc comparisons revealed that the keyhole was significantly stronger than the RCI screw (P =.033) but not significantly different compared with the Sysorb screw (P =.129). No significant difference was observed between the Sysorb and RCI screws (P =.762). Interference screw fixation failed by tendon slippage at the screw-tendon-bone interface; keyhole fixation failed by tendon splitting and slippage out of the restraining keyhole. Keyhole tenodesis may permit earlier postoperative mobilization when compared with tenodesis by use of interference screw fixation.

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.

Ultimate tensile strength of a Leeds-Keio/autograft ACL reconstruction utilizing PLLA tibial staple fixation.

An ovine model of anterior cruciate ligament reconstruction was utilized to evaluate the biomechanical and histological response of a polylactic acid tibial fixation staple (Gunze Ltd., Japan/Zimmer, Japan). This was performed in a comparison with metallic staples, currently utilized for this procedure. The prosthesis consisted of autograft combined with a Leeds-Keio (L-K) ligament. Early post-operative mobilization was followed by retrieval of specimens at 6, 12 and 24 weeks post-reconstruction. Evaluation of the mechanical characteristics of the graft reconstructions (tensile strength) showed no significant differences (P>0.05) between the staple types for each time period. The histological response to the polylactic acid staple was minimal over the time period studied, with no adverse tissue reactions observed. The mode of reconstruction failure was observed to change with time (P<0.05) presumably as the graft characteristics alter. Overall the absorbable staples performed at a comparable level with the metallic staples within the scope of the study.