Edward L. Milne

Precision oblique osteotomy for shortening of the ulna

The obstacles of prolonged healing time and technically demanding osteotomy and plate fixation in the performance of ulnar shortening osteotomies have been overcome by a precision system that includes a 45 degrees osteotomy and 2.7 mm interfragmentary lag screw. In 23 transverse osteotomies healing time averaged 21 weeks with one nonunion. In 17 precision oblique osteotomies healing time averaged a substantially shorter 11 weeks. Biomechanical data obtained from cadaveric testing comparing these two constructs demonstrated a structural stiffness that was clearly greater in torsion testing for the oblique osteotomy. No biomechanical difference was identified in the anteroposterior and lateral bending tests. The system permits the reliable performance of two parallel osteotomy cuts, allowing the removal of a precise amount of bone. The compression device and specialized plate permit easy coaptation of the osteotomy surfaces, which are locked into position by a precise 22 degrees interfragmentary lag screw. The surgical procedure is more quickly completed, and the frustration of this previously challenging procedure is now completely removed.

Mechanical failures of intramedullary tibial nails applied without reaming

The clinical mechanical failures of small diameter intramedullary interlocking nails were evaluated to determine the relationship of failure modes to the type or location of tibial fractures. Methods were developed to duplicate failure modes in vitro in standardized tests to simulate the clinical situations. Where standard test methods were inadequate, new methods were developed to provide quantifiable, reliable methods of evaluating potential clinical performance. The modes and rates of mechanical failure in the clinical series were consistent among participating centers: (1) In diaphyseal fractures with secondary trauma, the intramedullary nail bent at the fracture site where the working length was unsupported; (2) failures that occurred several weeks after nailing were the result of fatigue fractures of the locking screws, usually at the distal end; and (3) nail and screw failures occurred most commonly in proximal and distal tibial fractures. The strength of the 8- and 9-mm sizes of Synthes and Russell-Taylor nails were comparable.

Sliding characteristics of compression hip screws in the intertrochanteric fracture: a clinical study.

Summary: Sliding of compression hip screws (CHS) is advantageous when it allows for controlled collapse of a hip fracture and progressive stabilization. A retrospective review was performed on 47 intertrochanteric (IT) fractures treated with a Zimmer CHS. Previous studies had identified certain parameters as being important to CHS sliding characteristics in vitro. Using conventional diagnostic material (radiographs), we sought to use these parameters as clinically useful tools. Screw sliding, as well as geometric parameters of CHS that relate to screw sliding, such as barrel engagement in relation to screw extension and screw-plate angle, were measured and calculated from serial radiographs. Almost all screw sliding occurred within 30 days postoperation. Fracture stability and quality of reduction were two main factors relating to screw sliding (p <0.01). High screw-plate angle and longer screw-barrel engagement had no correlation with screw sliding even in unstable fractures. All five failures were due to cut-out after complete or almost complete collapse of the sliding mechanism in non-anatomically reduced fractures in osteoporotic females. Unstable fractures in osteoporotic bone do seem to require supplementary fixation beyond sliding screw fixation alone. None of the mechanical parameters (as judged from plane radiographs) that control the tendency of the CHS to slide could be statistically correlated with incidence or degree of sliding. Therefore, it was concluded that it is impractical to attempt to predict sliding tendency from plane radiograph measurements.

Distribution of bone mineral density and bone strength of the proximal humerus

Clinical experience led us to the hypothesis that in the proximal humerus cancellous bone beneath the top part of the head is the strongest and the bone of the humeral neck is the weakest. This hypothesis was examined on dissected proximal humeri with bone mineral densitometry and an indention test. Both dual photon absorptiometry and bone mineral analyses confirmed that the top part of the humeral head was the region with the greatest amount of bone mineral. The humeral neck had approximately one half the bone mineral density of the humeral head. The cancellous bone of the neck had only one third the mechanical strength of the humeral head on the indention test. The high degree of osteoporosis of the neck region increases the difficulty of surgical treatment for displaced humeral neck fractures.

The Role of Soft Tissues in Plate Fixation of Proximal Phalanx Fractures

The tension band effect of plate fixation and the contribution of soft tissues to that effect was examined biomechanically in human proximal phalanges. Forty-six proximal phalanges in whole cadaver hands with all soft tissues in place (intact) and 43 proximal phalanges stripped of soft tissues (denuded) were tested. After midshaft osteotomy, each proximal phalanx was fixed internally with a dorsal minicondylar plate, a lateral minicondylar plate, a dorsal straight plate, or a lateral straight plate. Specimens were tested in three-point apex dorsal bending to clinical failure, defined as 30° angulation. Ultimate moment (stability) at this angulation was similar among the four fixation methods in the specimens with all soft tissues intact. Stability also was similar among these methods in the denuded specimens. There were no significant differences in stability between minicondylar and straight plates or between dorsal and lateral plates in the specimens with soft tissues, nor were there significant differences between these groups in the denuded specimens. The stability of the four fixation methods was significantly greater in the specimens with soft tissues than in the denuded specimens. Soft tissues increased the stability of lateral minicondylar plates by 163%, lateral straight plates by 157%, dorsal minicondylar plates by 126%, and dorsal straight plates by 104%, providing a dorsal tension band effect that counteracted the buttress (compression) of the volar fracture surfaces of the phalanx. The results suggest that in the clinical setting a laterally placed straight or minicondylar plate may provide as much stability to a phalanx with a midshaft fracture as does the traditional, more invasive dorsally placed minicondylar or straight plate. These findings must be evaluated with caution, however, because all specimens were from embalmed cadavers, and the formalin fixation may have augmented the stability and stiffness of the soft tissues in the intact specimens. A subsequent pilot study comparing intact proximal phalangeal specimens that were formalin-fixed with those that were fresh-frozen showed a significant increase in stability and stiffness of formalin-fixed specimens.

A new modular testing system for biomechanical evaluation of tibial intramedullary fixation devices

This biomechanical study was performed to evaluate a new modular, tibial testing system developed for analysis of tibial nails and their locking screws.A new testing system, consisting of five modules, was designed to simulate a tibia. For this study one module was removed to simulate a 55-mm distal tibial defect inducing maximum loading on the distal portion of the implant and locking bolts. The tibial load offsets were 23 mm proximally and 10 mm distally medial to the centreline of the tibial shaft to simulate the location of the expected resultant load during the peak loading and inversion torque on the ankle during the gait cycle. Four solid tibial nails (STN, Stryker-Howmedica-Osteonics, Kiel, Germany) were tested to static failure and 15 nails were tested dynamically. Our results showed that the solid tibial nails fractured in the testing device in the same manner and location as they do in clinical series. Evaluation of the results showed the mean fatigue limit of the STN to be 1.4 kN for 500,000 cycles with a standard deviation (S.D.) of 0.33 kN. This biomechanical study establishes a standard technique for the biomechanical testing of tibial nails, in a clinically relevant manner, avoiding the inconsistency of cadaver bone tests. As it is a standardised test set-up this new modular testing system could serve as a standard by which small diameter tibial nails and other devices could be evaluated and compared with other systems currently in use.

An absence of structural changes in the proximal femur with osteoporosis

The hypothesis that osteoporosis occurs not as a preferential loss of the tensile trabeculae but as a general loss of bone was tested by using bone mineral densitometry and an indention test on dissected proximal femora. As osteoporosis advanced a significantly correlated decrease was found in both bone mineral density and mechanical properties between the principal compressive and tensile trabeculae. The decrease correlated with a decrease in the Singh index. These findings led to the conclusion that a sequential bone loss from the tensile trabeculae to the compressive ones did not occur as Singh reported, but instead a generalized loss of bone mineral in both the tensile and compressive trabeculae supervened. The structural changes, on which the grading system by Singh was based, were not observed in the proximal femur affected by osteoporosis.

Role of soft tissues in metacarpal fracture fixation.

The contribution of soft tissues in stabilizing fracture fixation in metacarpals is appreciated clinically, but no quantitative biomechanical study of their role has been done. All previous studies of fracture fixation in vitro have been done on metacarpals denuded of soft tissues. To quantify the role of soft tissues in metacarpal fracture fixation, the biomechanical effectiveness of four fixation devices was examined in human cadaver metacarpals with and without soft tissues. Values were compared for three nonrigid methods (expandable intramedullary fixation devices, crossed Kirschner wires, and single half-pin frames) and one rigid method (dorsal plates) in 45 disarticulated metacarpals stripped of soft tissues (denuded) and in 46 metacarpals in whole hands with all soft tissues remaining (intact). Mechanical testing to complete failure in three-point apex dorsal bending was done in all specimens. Ultimate moment (strength) of each of the four fixation methods was significantly greater in intact specimens than in denuded specimens. Crossed Kirschner wires were most stable in intact specimens, and dorsal plates were more stable in denuded specimens. The results show that soft tissues contribute to the strength of fracture fixation. Clinically, surgeons may be able to use a less invasive fixation method than plating without compromising the strength of metacarpal fixation in patients whose soft tissues are not severely disrupted and the fracture configuration allows. Plating may offer optimum stability in patients whose soft tissues are damaged severely and provide less strengthening of the fracture construct.

Effects of gamma irradiation on the biomechanical properties of peroneus tendons

Purpose This study was designed to investigate the biomechanical properties of nonirradiated (NI) and irradiated (IR) peroneus tendons to determine if they would be suitable allografts, in regards to biomechanical properties, for anterior cruciate ligament reconstruction after a dose of 1.5–2.5 Mrad. Methods Seven pairs of peroneus longus (PL) and ten pairs of peroneus brevis (PB) tendons were procured from human cadavers. The diameter of each allograft was measured. The left side of each allograft was IR at 1.5–2.5 Mrad, whereas the right side was kept aseptic and NI. The allografts were thawed, kept wet with saline, and attached in a single-strand fashion to custom freeze grips using liquid nitrogen. A preload of 10 N was then applied and, after it had reached steady state, the allografts were pulled at 4 cm/sec. The parameters recorded were the displacement and force. Results The elongation at the peak load was 10.3±2.3 mm for the PB NI side and 13.5±3.3 mm for the PB IR side. The elongation at the peak load was 17.4±5.3 mm for the PL NI side and 16.3±2.0 mm for the PL IR side. For PL, the ultimate load was 2,091.6±148.7 N for NI and 2,122.8±380.0 N for IR. The ultimate load for the PB tendons was 1,485.7±209.3 N for NI and 1,318.4±296.9 N for the IR group. The ultimate stress calculations for PL were 90.3±11.3 MPa for NI and 94.8±21.0 MPa for IR. For the PB, the ultimate stress was 82.4±19.0 MPa for NI and 72.5±16.6 MPa for the IR group. The structural stiffness was 216.1±59.0 N/mm for the NI PL and 195.7±51.4 N/mm for the IR side. None of these measures were significantly different between the NI and IR groups. The structural stiffness was 232.1±45.7 N/mm for the NI PB and 161.9±74.0 N/mm for the IR side, and this was the only statistically significant difference found in this study (P=0.034). Conclusion Our statistical comparisons found no significant differences in terms of elongation, ultimate load, or ultimate stress between IR and NI PB and PL tendons. Only the PB structural stiffness was affected by irradiation. Thus, sterilizing allografts at 1.5–2.5 Mrad of gamma irradiation does not cause major alterations in the tendons’ biomechanical properties while still providing a suitable amount of sterilization for anterior cruciate ligament reconstruction.

Biomechanical evaluation of the cylindrical TI mesh cage for the treatment of segmental defects of the tibia

Introduction: A recent report showed successful management of large diaphyseal segmental bone defects with titanium mesh cages and intramedullary nails. The purpose of the study was a quantitative evaluation of the of the mechanical role of a cylindrical titanium mesh cage and an intramedullary nail when used for the treatment of significant segmental defects of the tibia. Methods: : 28 adult rats after sustaining closed, mid shaft femur fracture, were bled to maintain a mean arterial pressure of 40 mmHg for 20 minutes while monitoring blood oxygen content. Groups: 1) no therapy; 2) 15 ml/kg Hextend infusion over 40 minutes; 3) 3 mg/kg BIIB513 (NHE-1 inhibitor) + 15 ml/kg Hextend infusion over 40 minutes. After 4 hours, the animals who survived received a second infusion of Hextend. Results: All animals in the no therapy group died within 2 hours. Group 3 showed improved hemodynamic response to fluid resuscitation, increased blood oxygen content, prevented metabolic acidosis, and improved 6 hour survival, 42%, compared to group 3, 80%. NHE-1 inhibition also resulted in reduced plasma levels of TNF-α, ICAM-1 and C-reactive protein, and attenuated neutrophil infiltration in the liver. There were no morhpologic changes found in any of the tissue samples at this early time period. Conclusion: NHE inhibition prevented early death caused by cardiac arrest, and multiple organ failure by inhibiting neutrophil infiltration and NF-(B activation, thereby, reducing systemic inflammation in a model of severe hemorrhagic shock and fracture in a rat.