Transverse fractures of the olecranon: a biomechanical comparison of three fixation techniques.

INTRODUCTION: The gold standard for treating transverse olecranon fractures is tension band fixation. A problem with this technique is migration of the K-wires leading to premature hardware removal. The aim of this study is to compare stability provided by two new techniques designed to eliminate the problem with backing out of K-wires, with that of the recommended tension band technique, performed with a biomechanical in vitro investigation. Our hypothesis was that the two new techniques would provide at least equal stability as the traditional tension band fixation. METHODS: Transverse olecranon osteotomies were created in human cadaveric elbows to simulate a type 21-B1.1 fracture. Three groups of 8 specimens were instrumented with: (1) recommended AO tension band technique; (2) modified K-wires with eyelets and tension band; (3) staples across the fracture with tension band. Each elbow was tested in a 90° flexed position. The triceps tendon was sinusoidally loaded by applying two load steps at 500 and 700 N for 4000 cycles each. Relative movements between the fragments were determined. RESULTS: At the end of the first and second load step the displacement of the osteotomy at the posterior ulnar side was significantly less for the staples across the fracture with tension band as compared to both other groups. There were no significant differences between groups 1 and 2. CONCLUSION: Since clinical results depend partly on stable fixation, it is concluded that using staples in the clinical situation might provide better results than the currently recommended tension band technique.

Three-dimensional distribution of trabecular bone density and cortical thickness in the distal humerus.

BACKGROUND: One major barrier to osteosynthesis in distal humeral fractures is poor bone quality. This study was an attempt to measure the bone quality in the distal humerus. METHODS: We measured the distribution of total bone mineral density (BMD), trabecular BMD (tBMD), and cortical thickness (CTh) in the distal humerus using peripheral quantitive computed tomography. Four slices in the infracondylar, supracondylar, and distal disphyseal regions of 25 human cadaver humeri were investigated. RESULTS: Total BMD decreased continuously from the distal diaphysis to the trochlea. Within the infracondylar region, the capitellum was the region of lowest tBMD and CTh (P < .001). Measurements in anterior regions were higher than in most others (P < .001). The tBMD of the medial column in the infracondylar and supracondylar regions was 31% and 36% higher vs the lateral column (P < .001). The medial column had an average 22% higher CTh in the supracondylar and 38% higher CTh in distal diaphyseal regions vs the lateral sides (P < .001). CONCLUSIONS: Distal humeral bone properties vary widely, providing stronger bone stock on the medial side. This may improve understanding of implant failure and techniques in surgical treatment.

Lateral insertion points in antegrade femoral nailing and their influence on femoral bone strains.

OBJECTIVES: Insertion of rigid uniplane bent femoral nails through the piriform fossa has been reported to cause neurovascular complications. New nails were designed for more lateral entry points. However, these may be associated with a higher risk of iatrogenic fractures. This study investigated if two differently bent nails with more lateral entry points induce higher cortical bone strains than a uniplane bent nail introduced through the piriform fossa. METHODS: Three groups of 8 cadaveric femurs were instrumented using the following nail systems and entry points: Cannulated Femoral Nail, piriform fossa; Antegrade Femoral Nail, trochanteric tip; and helical nail, lateral of the trochanteric tip. During insertion, the maximum principal bone strains were recorded at 9 locations at the proximal femur and the diaphysis. The occurrence of iatrogenic fractures or fissures was documented. RESULTS: The highest strains recorded were between 2000 and 4500 mum/m and mainly located at the posterior aspect of the greater trochanter and at the medial side of the entry point. In most of these cases fissures or fractures occurred, the number of which was higher for the trochanteric tip group as compared with the other groups. This was thought to be due to the thin cortical walls as a result of the larger reamer diameter in this group. Low strains (below 2000 microm/m) occurred at the medial cortex where the laterally inserted nails were expected to impinge. CONCLUSIONS: Bone strains at the medial impingement location were low for all nails. Entry portals with thin cortical walls due to, for example, larger reamer diameters and a small greater trochanter seem to be more susceptible to insertion accuracy, which may influence strain and fissure or fracture occurrence. Furthermore, we do not recommend determination of the entry point of laterally inserted nails based solely on anatomic landmarks of the greater trochanter because this may influence insertion accuracy. This implies that biplanar imaging is important for accurate and safe insertion of laterally started nails.

Mechanical comparison in cadaver specimens of three different 90-degree double-plate osteosyntheses for simulated C2-type distal humerus fractures with varying bone densities.

OBJECTIVES: To investigate the bone-implant-anchorage of 90-degree double-plate osteosynthesis in simulated complete intra-articular distal humerus fractures using conventional reconstruction plates (CRP), locking compression plates (LCP), and distal humerus plates (DHP), depending on the bone mineral density (BMD) of the cadaver specimens. METHODS: Groups (CRP, LCP, DHP, n=8; LCP, DHP, n=13) in distal humerus cadaver bones were created based on BMD. The fracture model was an unstable intraarticular distal humerus fracture with a transverse osteotomy gap representing metaphyseal comminution (AO type 13-C2.3). Flexion and extension stiffness as well as cycles until failure due to screw pullout under cyclic loading were evaluated. Estimates of BMD values, below which failure was likely to occur, were determined. RESULTS: Stiffness values were not significantly different between groups (extension: P=0.881, flexion: P=0.547). Under cyclic loading, consistent screw pullout failure occurred at BMD values below about 400 mg/cm for CRP and below about 300 mg/cm for LCP constructs. Comparing BMD-matched groups of 8 and 13 specimens respectively, the failure rate was significantly lower for the DHP (0/8) than for the CRP (5/8; P=0.026) and tended to be lower for the DHP (0/13) as compared to the LCP (4/13; P=0.096). CONCLUSION: Bone-implant anchorage was different between locking and nonlocking plate constructs and depended on BMD. While in good bone quality implant choice was not critical, both locking plates provided superior resistance against screw loosening as compared to the CRP at low BMD values (<420 mg/cm). Based on our laboratory results, we conclude that locking plates such as the LCP and DHP are constructs designed to keep anatomical reduction in the presence of comminution and poor bone quality in a low intra-articular fracture of the distal humerus.

Biomechanical testing of different osteosynthesis systems for segmental resection of the mandible.

PURPOSE: This investigation assessed the mechanical behavior of 3 different locking and nonlocking reconstruction systems-Unilock 2.4, Reconstruction 2.4, and Reconstruction 2.7-with regard to plate and screw fracture. MATERIALS AND METHODS: Five different plate/screw configurations (Unilock 2.4-locking screws, Unilock 2.4 -conventional screws, Reconstruction 2.4-conventional screws, Reconstruction 2.7-conventional screws, and Unilock 2.4-locking screws with a 1-mm gap; Synthes, Umkirch, Germany) were tested on synthetic mandibles. All mandibles were resected on the left side between the canine and third molar, reconstructed, and loaded cyclically between 30 and 300 N up to 250,000 cycles or until screw or plate failure occurred. RESULTS: No screw fractures were observed. All plates fractured close to the distal fragment. The Unilock plates fixed with locking screws withstood significantly more cycles until failure than the Reconstruction plates 2.4 fixed with conventional MF-Cortex screws. No significant differences were found in the other groups. Only 2 of the 34 plates tested, both of the Reconstruction 2.7 system, reached the runout limit. CONCLUSIONS: Unilock plates fixed with locking screws have a higher long-term stability than the Reconstruction 2.4 system. A 1-mm gap between the plate and mandible does not lead to early screw failure in the Unilock 2.4 system with locking screws. The Reconstruction 2.7 system seems superior if well contoured, because 2 of those plates reached the runout limit; however, this system is not as easy to handle as the 2.4 systems, and good contouring is difficult to achieve. Therefore, we consider the Unilock 2.4 system with locking screws the best choice.

Pullout strength of anterior spinal instrumentation: a product comparison of seven screws in calf vertebral bodies.

A lot of new implant devices for spine surgery are coming onto the market, in which vertebral screws play a fundamental role. The new screws developed for surgery of spine deformities have to be compared to established systems. A biomechanical in vitro study was designed to assess the bone-screw interface fixation strength of seven different screws used for correction of scoliosis in spine surgery. The objectives of the current study were twofold: (1) to evaluate the initial strength at the bone-screw interface of newly developed vertebral screws (Universal Spine System II) compared to established systems (product comparison) and (2) to evaluate the influence of screw design, screw diameter, screw length and bone mineral density on pullout strength. Fifty-six calf vertebral bodies were instrumented with seven different screws (USS II anterior 8.0 mm, USS II posterior 6.2 mm, KASS 6.25 mm, USS II anterior 6.2 mm, USS II posterior 5.2 mm, USS 6.0 mm, USS 5.0 mm). Bone mineral density (BMD) was determined by quantitative computed tomography (QCT). Failure in axial pullout was tested using a displacement-controlled universal test machine. USS II anterior 8.0 mm showed higher pullout strength than all other screws. The difference constituted a tendency (P = 0.108) when compared to USS II posterior 6.2 mm (+19%) and was significant in comparison to the other screws (+30 to +55%, P < 0.002). USS II posterior 6.2 mm showed significantly higher pullout strength than USS 5.0 mm (+30%, P = 0.014). The other screws did not differ significantly in pullout strength. Pullout strength correlated significantly with BMD (P = 0.0015) and vertebral body width/screw length (P < 0.001). The newly developed screws for spine surgery (USS II) show higher pullout strength when compared to established systems. Screw design had no significant influence on pullout force in vertebral body screws, but outer diameter of the screw, screw length and BMD are good predictors of pullout resistance.

Influence of screw positioning in a new anterior spine fixator on implant loosening in osteoporotic vertebrae.

STUDY DESIGN: A biomechanical study was designed to assess implant cut-out of three different angular stable anterior spinal implants. Subsidence of the implant relative to the vertebral body was measured during an in vitro cyclic loading test. OBJECTIVES: The objective of the study was to evaluate two prototypes (Synthes) of a new anterior spine fixator with different screw angulations in comparison to the established MACSTL(R) Twin Screw Concept (Aesculap). The influence of factors like load-bearing cross-sectional area, screw angulation and bone mineral density upon implant stability should be investigated. SUMMARY OF BACKGROUND DATA: Epidemiologic data predict a growing demand for appropriate anterior spinal fixation devices especially in patients with inferior structural and mechanical bone properties. Although different concepts for anterior spinal instrumentation systems have been tried out, implant stability is still a problem. METHODS: Three angular stable, anterior spinal implants were tested using 24 human lumbar osteoporotic vertebrae (L1-L5; age 84 (73-92)): MASC TL system (Aesculap); prototype 1 (MP1) with 18 degrees and prototype 2 (MP2) with 40 degrees screw angulation (both Synthes). All implants consisted of two screws with different outer screw diameters: 7-mm polyaxial screw with 6.5-mm stabilization screw (MASC TL), two 5-mm locking-head screws each (MP1 and MP2). Bone mineral density (BMD) and vertebral body width of the three specimen groups were evenly distributed. The specimens were loaded in craniocaudal direction (1Hz) for 1000 cycles each at three consecutive load steps; 10-100 N, 10-200 N and 10-400 N. During cyclic loading subsidence of the implant relative to the vertebral body was measured in the unloaded condition. Cycle number at failure (defined as a subsidence of 2 mm) was determined for each specimen. A survival analysis (Cox Regression) was performed to detect differences between implant groups at a probability level of 95%. RESULTS: High correlations were found between BMD and number of cycles until failure (MP1; r = 0.905, P = 0.013; MP2: r = 0.640, P = 0.121; MACS TL: r = 0.904, P = 0.013) and between load bearing cross sectional area and number of cycles until failure (MP1: r = 0.849, P = 0.032;MP2: r = 0.692, P = 0.085; MACS TL: r = 0.902, P = 0.014). Both Prototypes survived significantly longer than the MACS TL implant (MP1: P = 0.012, MP2: P = 0.014). The survival behaviour of MP1 and MP2 was not significantly different (P = 0.354). CONCLUSIONS: Implant stability within each implant group was influenced by BMD and load bearing cross-sectional area. The angulation of the two screws did not have a significant influence on cut-out. As conclusion from this study, promising approaches for further implant development are: 1) increase of load-bearing cross-sectional area (e.g., larger outer diameter of the anchorage device), 2) screw positioning in areas of higher BMD (e.g., opposite cortex, proximity to pedicles or the endplates).

Assessment of bone quality in the proximal humerus by measurement of the contralateral site: a cadaveric analyze.

INTRODUCTION: The presence of osteoporosis decreases the success of osteosynthesis, especially in the proximal humerus. Estimation of the bone mineral density (BMD) at the fracture site could aid in the decision making for surgical treatment and potential implant choice with regard to the individual bone properties. BMD measurement at a fracture site is prone to inaccuracies and alternative measurement sites need to be identified. In the case of a proximal humerus fracture, promising alternative measurement sites are at the same contralateral or at a different ipsilateral location. The aim of this study was to determine if the BMD for the humeral head can be predicted by BMD measurements from the ipsilateral distal humerus or the contralateral proximal humerus. MATERIAL AND METHODS: Cancellous BMD values were obtained from 88 paired human cadaver humeri (age 75.8+/-13.5 years) at the humeral head and at the distal metaphyseal area by pQCT. Correlations between BMD values of the ipsi- and contralateral sites were computed. RESULTS: Correlations between proximal and distal BMD values within one bone were moderate for both left (R2=0.37) and right humeri (R2=0.40). BMD comparison between left and right humeri revealed high correlations for both the distal (R2=0.90) and the proximal humerus (R2=0.74) (all P<0.01). Elderly specimen (>or=70 years) showed better intersite correlations between all regions than younger specimen (<70 years). CONCLUSION: High correlations between contralateral BMD values may be the result of similar biomechanical loading conditions. Although a relationship between proximal and distal bone quality of the same bone was found, the moderate coefficient suggests that ipsilateral measurements do not provide a good prediction of humeral head BMD. Bone quality at the humeral head is best predicted by BMD measurements at the contralateral location rather than the ipsilateral distal site.

Assessment of stiffness and strength of 4 different implants available for equine fracture treatment: a study on a 20 degrees oblique long-bone fracture model using a bone substitute.

OBJECTIVE: To compare the mechanical properties of 4 stabilization methods for equine long-bone fractures: dynamic compression plate (DCP), limited contact-DCPlate (LC-DCP), locking compression plate (LCP), and the clamp-rod internal fixator (CRIF--formerly VetFix). STUDY DESIGN: In vitro mechanical study. SAMPLE POPULATION: Bone substitute material (24 tubes) was cut at 20 degrees to the long axis of the tube to simulate an oblique mid-shaft fracture. METHODS: Tubes were divided into 4 groups (n=6) and double plated in an orthogonal configuration, with 1 screw of 1 implant being inserted in lag fashion through the "fracture". Thus, the groups were: (1) 2 DCP implants (4.5, broad, 10 holes); (2) 2 LC-DCP implants (5.5, broad, 10 holes); (3) 2 LCP implants (4.5/5.0, broad, 10 holes) and 4 head locking screws/plate; and (4) 2 CRIF (4.5/5.0) and 10 clamps in alternating position left and right of the rod. All constructs were tested in 4-point bending with a quasi-static load until failure. The implant with the interfragmentary screw was always positioned on the tension side of the construct. Force, displacement, and angular displacement at the "fracture" line were determined. Construct stiffness under low and high loads, yield strength, ultimate strength, and maximum angular displacement were determined. RESULTS: None of the implants failed; the strength of the bone substitute was the limiting factor. At low loads, no differences in stiffness were found among groups, but LCP constructs were stiffer than other constructs under high loads (P=.004). Ultimate strength was lowest in the LCP group (P=.01), whereas yield strength was highest for LCP constructs (409 N m, P=.004). CRIF had the lowest yield strength (117 N m, P=.004); no differences in yield strength (250 N m) were found between DCP and LC-DCP constructs. Differences were found for maximum angular displacement at the "fracture" line, between groups: LPC

Range of motion in reconstruction situations following corpectomy in the lumbar spine: a question of bone mineral density?

STUDY DESIGN: In vitro biomechanical study to evaluate the stability of different types of instrumentation in the lumbar spine following corpectomy in relation to bone mineral density (BMD). OBJECTIVES: To investigate the relation between the stability of a spinal instrumentation and BMD. To determine a threshold value of BMD allowing a single ventral instrumentation following corpectomy in the lumbar spine. SUMMARY OF BACKGROUND DATA: Some in vitro studies determined the biomechanical properties of different spinal instrumentations in various spinal injury models. To the authors' knowledge, there are no published data available concerning stabilization in relation to BMD. A guideline for the treatment of a corpectomy depending on BMD would be helpful in order to choose the appropriate surgical method. METHODS: Twenty-four fresh frozen human lumbar cadaveric spine specimens L1-L3 were used for testing of biomechanical properties. Plain radiographs were taken. BMD was determined using quantitative computed tomography (QCT). Testing in a 6 df loading device included native specimens and specimens after corpectomy of L2, restoration of the defect with a titanium cage, and two reconstruction situations: single ventral and additional dorsal instrumentation. Load-displacement curves and range of motion parameters were recorded and correlated with BMD. RESULTS: A significant (P < 0.05) influence of BMD on range of motion was found. Single ventral instrumentation was critical concerning axial rotation. Combined dorsoventral instrumentation offered sufficient stability. The threshold value for use of single ventral instrumentation is a BMD > or = 0.22 g/cm. CONCLUSIONS: Single ventral instrumentation can provide sufficient stability following corpectomy in the lumbar spine under the condition of a high BMD. Determination of BMD and the use of this guideline provides a valid tool for surgical planning.

Treatment strategies for proximal femur fractures in osteoporotic patients.

Fractures of the proximal end of the femur, together with vertebral fractures, are greatly exacerbated by osteoporosis and can be regarded as the most typical and most serious complications of this disease. The demand for prompt mobilisation with full loading of the affected limb, combined with a desire for the gentlest of treatments, becomes increasingly difficult to meet in ageing patients with advanced osteoporosis. The advantages of osteosynthesis in respect to these demands when operating on elderly patients with fractures do not apply due to the inability of the osteoporotic bone to hold the osteosynthetic components sufficiently until fracture healing occurs. This inability is related to the anatomy of the proximal end of the femur and its loading patterns. Under eccentric loading, high bending loads occur, leading to failure of the osteosynthetic anchorage at the center of the femoral head. This leads subsequently to stressful revision operations for the patient. The prosthetic replacement is a good option in cases of dislocated intracapsular fractures, but in cases of trochanteric fractures it is still debated. Therefore, it is vital for the trauma surgeon to have specific knowledge of the patient's bone quality in order to optimise the result of the preferred procedure. With reference to our own experimental research and a study of the current literature, this knowledge can be summarised as follows: the most stable anchorage for the implant is achieved by placing the implant through the midpoint of the femoral head (highest bone mineral density) or just below ("best backing"). Anchoring femoral head implants so that they are stable in rotation within the head-neck fragment will significantly raise their load bearing capacity. This is also true for intramedullary load bearing devices in trochanteric fractures. The distance between the load-bearing device in the femoral neck and the articular surface is inversely correlated to the stability at yield, as other studies have already shown. There seems to be a limit for a successful realisation of an osteosynthesis that lies at a femoral head bone mineral density of 250 mg/cm(3) calcium-hydroxyapatite (CaHAp). Nevertheless, high precision surgery in regard to fracture reduction and implant placement is a essential requirement for a successful osteosynthesis. Reproducible local measurements of bone mineral density and trabecular alterations, as well as quick screening methods, are very much desired by the authors.

Biomechanical analysis of transpedicular screw fixation in the subaxial cervical spine.

STUDY DESIGN: An in vitro biomechanical study to compare 2 different dorsal screw fixation techniques in the cervical spine with respect to primary stability and stability after cyclic loading. OBJECTIVES: To investigate if the biomechanical stability is better in pedicle screw or in lateral mass fixation. SUMMARY OF BACKGROUND DATA: In patients with poor bone quality who require multisegmental fixations, the current dorsal stabilization procedures in the subaxial cervical spine using lateral mass screws are often insufficient. Cervical pedicle screw fixation has been suggested as an alternative procedure, but there are still limited data available on the biomechanical differences between pedicle screw and lateral mass fixation. METHODS: A severe multilevel discoligamentous instability was created in 8 human cervical spine specimens (C2-C7). Dorsal stabilization was performed with the assistance of computer navigation (SurgiGate, Medivison, Switzerland) using either lateral mass or pedicle screw fixation. In the first part of the study, primary stability was measured by means of a multidirectional flexibility test. Then, specimens were divided into 2 groups, randomized for bone mineral density. Cyclic loading was applied with sinusoidal loads in flexion/extension (1000 cycles, +/-1.5 Nm, 0.1 Hz). Mechanical behavior of the specimens was determined by a flexibility test before and after the application of cyclic loads. Data analysis was performed by calculating the ranges of motion, and statistical differences were determined with the t test for group comparison. RESULTS: Pedicle screw fixation showed a significantly higher stability in lateral bending (pedicle screw range of motion 0.86 +/- 0.31 degrees; lateral mass range of motion 1.43 +/- 0.62 degrees; P = 0.037). No significant differences were seen in flexion/extension and axial rotation. After cyclic loading, the decrease in stability was less with pedicle screw fixation in all load directions. Differences in the decrease of stability were statistically significant in flexion/extension (pedicle screw 95.4 +/- 9.4%; lateral mass 70.5 +/- 9.8%; P = 0.010) and lateral bending (pedicle screw 105.3 +/- 5.0%; lateral mass 84.2 +/- 13.6%; P = 0.046), whereas there was no significant difference in axial rotation. CONCLUSIONS: The major finding of the current study was the higher stability of pedicle screws over lateral mass fixation with respect to primary stability and stability after cyclic loading. From a biomechanical point of view the use of pedicle screws in the subaxial cervical spine seems justified in patients with poor bone quality and need for multisegmental fixation.

A biomechanical evaluation of methods of distal humerus fracture fixation using locking compression plates versus conventional reconstruction plates.

OBJECTIVES: To examine the biomechanical behavior of 2 techniques of double-plate osteosynthesis for fractures of the adult distal humerus using conventional reconstruction plates and locking compression plates. DESIGN: Basic science study. SETTING: Experimental in vitro study. PATIENTS/PARTICIPANTS: Forty fresh-frozen human distal humeri specimens. INTERVENTION: Four matched groups with 10 humeri each, median age 74 years (46-95), were created using similar bone mineral density values. Two standard configurations of double-plate osteosynthesis (dorsal or 90 degrees configuration) with either conventional reconstruction plates or locking compression plates were studied for biomechanical properties of the constructs. A fracture model with a 5-mm supracondylar osteotomy gap simulating metaphyseal comminution (AO type 13-A3.3) was used. MAIN OUTCOME MEASUREMENT: Stiffness testing of the constructs in anterior/posterior bending, torsion, and axial compression loading. Evaluation of alterations of the bone-implant interface and failure patterns under cyclic loading and strength testing. RESULTS: The study demonstrates that primary stiffness in anterior/posterior bending and torsional loading is significantly increased by using locking compression plates in a 90 degrees configuration (P < 0.05) as compared with dorsally applied plates. The differences between the different plate types are insignificant if applied in the same configuration. It is demonstrated that none of the tested implants failed under cyclic loading within the number of cycles expected for 3 months of use. The bone-implant interface is less likely to fail during strength testing with locking compression plates. CONCLUSION: The biomechanical behavior of the osteosynthesis depends more on plate configuration than plate type. Advantages of locking compression plates are only significant if compared with dorsal plate application techniques. Nevertheless, locking compression plates are helpful supplementary tools for achieving primary stable fracture fixation. This might be of considerable clinical relevance in patients with diminished bone mineral quality or in the presence of metaphyseal comminution.

The LCP-concept in the operative treatment of distal humerus fractures--biological, biomechanical and surgical aspects.

Distal humerus fractures in adults are treated by open reduction and internal fixation, which produces good results in the majority of patients. However, in elderly patients or in cases with metaphyseal comminution, stable fracture fixation still remains problematic. Anatomical joint reconstruction and primary stable osteosynthesis are often particularly difficult to achieve in this group of patients since poor bone mineral quality is frequently encountered. Consequently, longer immobilization is necessary, which is known to negatively influence functional outcome. Over the past few years, double-plate osteosynthesis techniques using different configurations have become the treatment of choice. Nevertheless, complications due to inappropriate primary stability and/or implant failure have been described. Investigations on whether the recently introduced Locking Compression Plates (LCPs) could enhance primary stability are rare. On the basis of clinical and biomechanical experiences, the authors consider LCPs a helpful tool for increasing primary stability in osteosynthesis of distal humerus fractures. LCPs might be of substantial advantage in patients with diminished bone mineral quality or in the presence of metaphyseal comminution.

Mechanical comparison of 3 different clamp and 2 different rod types of a new veterinary internal fixation system, 4.5/5.5-mm VetFix.

OBJECTIVE: To compare the gripping force of a standard clamp with click-on and T clamps of the 4.5/5.5-mm VetFix (AO Research Institute, Davos, Switzerland), mounted on smooth and roughened rods. STUDY DESIGN: In vitro mechanical study. METHODS: Rods were mounted on a clamp-holding device. Each clamp type was tested on a materials-testing machine in groups of 6 specimens for torsional and axial load stability on smooth and roughened rods (total, 72 specimens). The variable evaluated was the discontinuity point (DP) as a measure for gliding resistance of the clamp on the rod. Means (+/-SD) were calculated for all groups separately. For statistical comparison, a multiple regression model was fitted with level of significance set at P =.05. RESULTS: The direction of torsion had a significant effect on DP for both asymmetric clamps (standard, click-on). In torsion and axial load, the roughened rod had significantly higher DP levels than the smooth rod for the standard clamp. Rods differed only in axial load for the T clamp, again favoring the roughened rod. The click-on clamp did not reach 10% of the values achieved by the other clamps, independent from the rods and loading conditions. Compared with the T clamp, standard clamp DP values were significantly higher in torsion on the roughened rod as well as in axial load on both rod types. CONCLUSIONS: The roughened rod with a standard clamp was mechanically superior. For the T clamp, the small loss of gripping force may be compensated for by the possibility of inserting 2 more screws. CLINICAL RELEVANCE: The roughened rod may enhance the bending, compressive, and torsional properties of the 4.5/5.5-mm VetFix system by reducing the risk of clamp gliding on the rod. The standard clamp is the clamp of choice, except for the metaphyseal or epiphyseal region, at which use of a T clamp may be indicated to increase the number of screws inserted at the end of the rod.

Biomechanical evaluation of the proximal femoral nail.

In 1997, the proximal femoral nail was introduced for treatment of peritrochanteric femoral fractures. Treatment results show a low complication rate. The most serious complication is cutout of the hip pin and femoral neck screw. Considerable load on the hip pin is thought to facilitate cutout. The biomechanical behavior of the hip pin and the femoral neck screw as part of the standard proximal femoral nail, and of an experimentally modified proximal femoral nail (in which the hole through the nail for the hip pin is modified to a slot) was studied. In the standard proximal femoral nail, the amount of the total load carried by the hip pin varies between 8% and 39% (mean, 21%). If the hip pin passes through a slot in the nail, it carries 2% to 8% (mean, 5%) of the load. The nonconstrained lateral end of the hip pin reduces the bending load applied to the implant. The slotted hole for the hip pin also allows the femur and the nail to medialize, even if the hip pin and femoral neck screw lose parallelism. The prevalence of cutout of the proximal femoral nail may be reduced by introduction of this mechanism.

Comparison of double dynamic compression plating versus two configurations of an internal veterinary fixation device: Results of in vitro mechanical testing using a bone substitute.

OBJECTIVE: To compare the mechanical properties of 2 configurations of a veterinary fixation system (VFS) for large animal long bones with dynamic compression plating (DCP). SAMPLE POPULATION: Eighteen pairs of Canevasit tubes (Canevasit; Amsler und Frei, Schinznach Dorf, Switzerland) (length, 170 mm; diameter, 47.5 mm; cortex thickness, 10 mm), aligned with a 10-mm gap, and stabilized with 2 DCP or 2 VFS implants. METHODS: Three groups (n = 6) were compared. Group 1 Canevasit tubes were stabilized with two 10-hole, broad 4.5-mm stainless steel DCP applied with both plates centered over the gap, in orthogonal planes parallel to the long axis of the tubes and staggered to allow bicortical fixation with ten 4.5-mm, 52-mm-long cortex screws each. Group 2 tubes were stabilized similarly with 2 VFS implants, each composed of a stainless steel rod (length, 167 mm; diameter, 8 mm), and 10 clamps were applied in alternating fashion left and right on the rod and fixed bicortically with ten 4.5-mm, 52-mm-long, cortex screws. Group 3 tubes were stabilized similarly, but using only 6 clamps/rod. All groups were tested initially in torsion within elastic limits and subsequently in 4-point bending, with 1 implant on the tension side, until gap closure occurred. RESULTS: None of the constructs failed, but all had plastic deformation after 4-point bending. No statistically significant differences were found among the 3 groups in torsional stiffness. Double DCP fixation was significantly stiffer and stronger in 4-point bending, compared with both configurations of double VFS fixation. CONCLUSIONS: The plate design was favored in this study. The VFS system may have to be adapted before further tests are conducted. Test modalities have to be chosen closer to clinical conditions (real bone, cyclic loading, closed gap). CLINICAL RELEVANCE: The veterinary fixation system has not yet proven its advantages for large animal long bone fracture repair. From the pure mechanical point of view, double DCP is the favored method for the treatment mentioned.