Mark Lenz

Ex vivo evaluation of the polymerization temperatures during cement augmentation of proximal femoral nail antirotation blades.

BACKGROUND: Previous studies have clearly demonstrated superior biomechanical behavior of augmented proximal femoral nail antirotation (PFNA) blades compared with nonaugmented ones with respect to implant cutout. Nevertheless, there is concern about thermal bone necrosis due to exothermic curing of polymethylmethacrylate (PMMA)-based bone cements. The objective of this study was to quantify the temperatures arising around perforated titanium PFNA blades when augmenting with PMMA. METHODS: Cylindrical samples from six pairs of fresh frozen human cadaveric femoral heads implanted with a PFNA blade were placed in a 37°C water bath and augmented with 3 mL and 6 mL PMMA. During augmentation, temperatures were measured using six K-type thermocouples that were placed at controlled distances around the implant. With the help of high-resolution quantitative computed tomography images, the locations of all thermocouples with respect to the cement-bone interface were reconstructed. RESULTS: No temperatures higher than 45°C were measured in the interface region and the surrounding cement-free cancellous bone. In the same regions, the longest exposure time above 41°C was 8.5 minutes and was measured in a 6-mL sample. Average maximum temperature was significantly lower for the 3-mL group compared with the 6-mL group (p = 0.017). CONCLUSION: The results of this study suggest that augmentation of titanium PFNA blades is not associated with a risk of thermal bone necrosis when using up to 6 mL of PMMA. However, larger amounts of cement lead to higher temperatures. PMMA application should therefore be kept low to minimally alter the biological system.

A biomechanical study on proximal plate fixation techniques in periprosthetic femur fractures

INTRODUCTION Proximal plate fixation is a crucial factor in osteosynthesis of periprosthetic femur fractures. Stability and strength of different fixation concepts for proximal plate fixation were compared. MATERIALS AND METHODS Twelve fresh frozen, bone mineral density matched human femora, instrumented with cemented hip endoprosthesis were osteotomized simulating a Vancouver B1 fracture. Specimens were instrumented with locking compression plates, fixed proximally with either locking attachment plate (LAP), monocortical screws, cerclage plus monocortical screws (1cerclage) or cerclages only (4cerclages). Cyclic testing was performed with monotonically increasing load until failure. Relative movements at the proximal plate-femur interface were registered by motion tracking. RESULTS The LAP construct exhibited a significantly longer cumulative survival (failure criterion 1mm separation at the proximal plate fixation) compared to the monocortical (p=0.048) and 4cerclages constructs (p=0.012) but not to 1cerclage constructs. CONCLUSION Bicortical screw anchorage improves proximal plate fixation in periprosthetic fractures. The cerclage-screw combination is a valuable alternative especially in osteoporotic bone.

Mechanical behavior of fixation components for periprosthetic fracture surgery

BACKGROUND Reliable periprosthetic fracture treatment needs detailed knowledge on the mechanical behavior of the fixation components used. The holding capacity of three conventional fixation components for periprosthetic fracture treatment was systematically investigated under different loading directions. METHODS Locking compression plates were fixed to a 7 cm long part of diaphyseal fresh frozen human femur with either a single 1.7 mm cerclage cable, a 5.0mm monocortical or a bicortical locking screw (n=5 per group). Constructs were loaded in lateral, torsional and axial direction with respect to the bone axis in a load-to-failure test. Corresponding stress distribution around the screw holes was analyzed by finite element modeling. FINDINGS Both screw fixations revealed significantly higher stiffness and ultimate strength in axial compression and torsion compared to the cerclage (all P<0.01). Ultimate strength in lateral loading and torsion was significantly higher for bicortical screws (mean 3968 N SD 657; mean 28.8 Nm SD 5.9) compared to monocortical screws (mean 2748 N SD 585; mean 14.4 Nm SD 5.7 Nm) and cerclages (mean 3001N SD 252; mean 3.2 Nm SD 2.0) (P≤0.04). Stress distribution around the screw hole varied according to the screw type and load direction. INTERPRETATION Fixation components may be combined according to their individual advantages to achieve an optimal periprosthetic fracture fixation.

Concept of variable angle locking--evolution and mechanical evaluation of a recent technology.

Applications for fracture‐adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°−15°), and locked at 0.8 Nm were subjected to a load‐to‐failure test. Ultimate failure moment at the screw‐head interface and failure mode of the screws were determined. Significant differences were detected by one‐way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°−10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head‐locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care.

Computational anatomy of the proximal humerus: An ex vivo high-resolution peripheral quantitative computed tomography study

Summary Background/Objective Spatial knowledge of the anatomy of the proximal humerus is critical for effective treatment, particularly in patients affected by fragility fractures. High-resolution peripheral quantitative computed tomography (HR-pQCT) imaging with medical image processing techniques enable three dimensional (3D) analysis of volumetric bone mineral density (vBMD) of bones of different sizes and shapes. Methods To elucidate the bony anatomy and to create 3D reference data, we conducted a computerized HR-pQCT-based study in intact postmortem samples of the proximal humerus to highlight the anatomy with particular emphasis on the size, shape, and bone stock distribution pattern. Fifty-eight defrozen intact humerus samples from 28 female and 30 male donors, who were aged 61–98 years old (mean age ± standard deviation, 80.6 ± 9 years), were scanned in the proximal third using the extended standard HR-pQCT protocol. A 3D statistical bone and averaged bone density models with low, middle, and high total vBMDs were computed. We examined the 3D patterns of size and shape variations using principal component analysis, and the vBMD distributions and variabilities using volume-rendering and virtual bore probing. Results The computer models revealed a highly variable bony anatomy in which size was the predominant variation in the first principal component (PC). In the second PC, we observed notable variabilities in the shape of the head and shaft inclination. A distinct 3D pattern of bone stock distribution was detected in which the lowest vBMD values were identified in the medullary cavity, middle values were identified in the central zone, and the highest values were identified in the cortex and humeral head—particularly in the subarticular zones. In the presence of bone loss, the vBMD values were ubiquitously decreased, but the pattern of 3D bone stock distribution was maintained. Conclusion The new anatomical 3D data that we acquired will improve the understanding of the normal bony anatomy of the proximal humerus. The extended HR-pQCT protocol and computer models may be used for other skeletal sites and used as 3D reference models that can be applied to systematically improve implant design and anchorage.

Wheelbarrow tire explosion causing trauma to the forearm and hand: a case report.

IntroductionTire explosion injuries are rare, but they may result in a severe injury pattern. Case reports and statistics from injuries caused by exploded truck tires during servicing are established, but trauma from exploded small tires seems to be unknown.Case presentationA 47-year-old german man inflated a wheelbarrow tire. The tire exploded during inflation and caused an open, multiple forearm and hand injury.ConclusionEven small tires can cause severe injury patterns in the case of an explosion. High inflating pressures and low safety distances are the main factors responsible for this occurrence. Broad safety information and suitable filling devices are indispensable for preventing these occurrences.

Enhancing fixation strength in periprosthetic femur fractures by orthogonal plating—a biomechanical study

Orthogonal plate osteosynthesis enhances fixation stability in periprosthetic femur fractures. Another option are locking attachment plates (LAP) allowing bicortical locking screw placement lateral to the prosthesis stem. Stability of lateral plate osteosynthesis with two LAP (2LAP) was compared to anterolateral orthogonal plate osteosynthesis (OP) with one LAP in a periprosthetic femur fracture model. In six pairs of fresh frozen human femora with cemented Charnley hip prosthesis, a transverse osteotomy was set distal to the tip of the prosthesis simulating a Vancouver type B1 fracture. Each pair was instrumented using a plate tensioner with either one lateral plate and two LAP, or two orthogonal anterolateral plates and one LAP. Stiffness was determined in a four‐point‐bending test prior to cyclic testing (2Hz) with physiologic profile and progressively increasing load up to catastrophic construct failure. Paired t‐test and Wilcoxon‐signed‐rank test were used for statistical evaluation at a level of significance p = 0.05. The OP construct exhibited a significantly higher number of cycles and load to failure (39,627 cycles ± 4,056; 4,463 N ± 906) compared to the 2LAP construct (32,927 cycles ± 3,487; 3,793 N ± 849), p < 0.01. Mediolateral bending and torsional stiffness of the OP (1610 N/mm ± 249; 16.9 Nm/mm ± 6.3) were significantly higher compared to 2 LAP (1077 N/mm ± 189; 12.1 Nm/mm ± 3.9), p = 0.03 for both comparisons. Orthogonal plate osteosynthesis is a valuable option in periprosthetic fracture surgery, offering increased stability compared to a single lateral plate fixed with two LAP.

The influence of the Peroneus Longus muscle on the foot under axial loading: A CT evaluated dynamic cadaveric model study.

BACKGROUND Subtle hypermobility of the first tarsometatarsal joint can occur concomitantly with other pathologies and may be difficult to diagnose. Peroneus Longus muscle might influence stability of this joint. Collapse of the medial longitudinal arch is common in flatfoot deformity and the muscle might also play a role in correcting Mearys angle. METHODS A radiolucent frame was used to simulate weightbearing during CT examination. Eight pairs fresh-frozen lower legs were imaged in neutral position under non-weightbearing (75N), weightbearing (700N) and with 15kg weights hung from Peroneus Longus tendon. Measurements included first metatarsal rotation, intermetatarsal angle, first tarsometatarsal joint subluxation and Mearys angle. FINDINGS Weightbearing significantly increased Mearys angle and significantly decreased first tarsometatarsal joint subluxation (both P<0.01). Pulling Peroneus Longus tendon significantly increased first metatarsal rotation (P<0.01), significantly decreased the intermetatarsal angle (P<0.01) and increased non-significantly Mearys angle (P=0.52). INTERPRETATION A considerable effect weightbearing has on the medial longitudinal arch and first tarsometatarsal joint was observed. Pulling Peroneus Longus tendon improved first metatarsal subluxation but increased its rotation. The study calls into question the importance of this tendon in maintaining the medial longitudinal arch and raises concerns about rotational deformity of the first metatarsal following hallux valgus correction without first tarsometatarsal arthrodesis. CLINICAL RELEVANCE Study outcomes will provide more insight in foot pathology. WHAT IS KNOWN ABOUT THE SUBJECT Weightbearing affects anatomy of the foot. No reliable information is available concerning the influence of the Peroneus muscle. WHAT THIS STUDY ADDS TO EXISTING KNOWLEDGE This study investigates the influence of weightbearing and the impact the Peroneus muscle on the anatomy of the foot.

Head-locking durability of fixed and variable angle locking screws under repetitive loading

Polyaxial locking screws are increasingly applied in fracture fixation. To investigate the durability of the head‐locking mechanism, the removal torque of variable angle (VA) and fixed angle (FA) stainless steel and titanium locking screws was investigated without and after a cyclic loading test. Stainless steel (St) and titanium (Ti) 2.4 mm orthogonally inserted FA screws and 2.4 mm VA screws inserted in different inclinations (0°–15°) (n = 6 per group) were locked at 0.8 Nm. Removal torque was determined without (W) and after (A) cyclic loading (sinusoidal load, 5 Hz, constant amplitude of 25 N, up to 10′000 cycles, or failure). Significant differences in‐between the groups were detected by Students t‐test (p < 0.05). Except VA Ti in 0deg and FA, all groups exhibited a drop in removal torque below the insertion torque without and after cyclic testing. The removal torque was (St: FA W:0.81 Nm ± 0.04 A:0.72Nm ± 0.04; VA0deg W:0.73 Nm ± 0.04 A:0.65 Nm ± 0.05; VA15deg W:0.51 Nm ± 0.05 A:0.50 Nm ± 0.08; Ti: FA W:0.82 Nm ± 0.03 A:0.70 Nm ± 0.04; VA0deg W:0.80 Nm ± 0.02 A:0.72 Nm ± 0.05; VA15deg W:0.55 Nm ± 0.03 A:0.54 Nm ± 0.06). In all groups, the removal torque after cyclic testing did not drop below 16% of the removal torque without cyclic testing. No head loosening was observed after cyclic testing. Stainless steel and titanium 2.4 mm fixed and variable angle locking screws provide a stable and lasting head‐locking mechanism.

Reconstruction of the lateral tibia plateau fracture with a third triangular support screw: A biomechanical study

Summary Background Split fractures of the lateral tibia plateau in young patients with good bone quality are commonly treated using two minimally invasive percutaneous lag screws, followed by unloading of the knee joint. Improved stability could be achieved with the use of a third screw inserted either in the jail-technique fashion or with a triangular support screw configuration. The aim of this study was to investigate under cyclic loading the compliance and endurance of the triangular support fixation in comparison with the standard two lag-screw fixation and the jail technique. Methods Lateral split fractures of type AO/OTA 41-B1 were created on 21 synthetic tibiae and subsequently fixed with one of the following three techniques for seven specimens: standard fixation by inserting two partially threaded 6.5 mm cannulated lag screws parallel to each other and orthogonal to the fracture plane; triangular support fixation—standard fixation with one additional support screw at the distal end of the fracture at 30° proximal inclination; and jail fixation—standard fixation with one additional orthogonal support screw inserted in the medial nonfractured part of the bone. Mechanical testing was performed under progressively increasing cyclic compression loading. Fragment displacement was registered via triggered radiographic imaging. Results Mean construct compliance was 3.847 × 10−3 mm/N [standard deviation (SD) 0.784] for standard fixation, 3.838 × 10−3 mm/N (SD 0.242) for triangular fixation, and 3.563 × 10−3 mm/N (SD 0.383) for jail fixation, with no significant differences between the groups (p = 0.525). The mean numbers of cycles to 2 mm fragment dislocation, defined as a failure criterion, were 12,384 (SD 2267) for standard fixation, 17,708 (SD 2193) for triangular fixation, and 14,629 (SD 5194) for jail fixation. Triangular fixation revealed significantly longer endurance than the standard one (p = 0.047). Conclusion Triangular support fixation enhanced interfragmentary stability at the ultimate stage of dynamic loading. However, the level of improvement seems to be limited and may not legitimate the intervention with an additional third screw.