Berend Linke

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° 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° 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.

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°; lateral mass range of motion 1.43 ± 0.62°; 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.

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/cm3 for CRP and below about 300 mg/cm3 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/cm3). 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.

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® 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° and prototype 2 (MP2) with 40° 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).

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 μm/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 μm/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.