Schuhli augmentation of plate and screw fixation for humeral shaft fractures: a laboratory study

Biomechanical Design Optimization of Distal Humerus Fracture Plates: A Review

The goal of this article was to review studies on distal humerus fracture plates (DHFPs) to understand the biomechanical influence of systematically changing the plate or screw variables. The problem is that DHFPs are commonly used surgically, although complications can still occur, and it is unclear if implant configurations are always optimized using biomechanical criteria. A systematic search of the PubMed database was conducted to identify English-language biomechanical optimization studies of DHFPs that parametrically altered plate and/or screw variables to analyze their influence on engineering performance. Intraarticular and extraarticular fracture (EAF) data were separated and organized under commonly used biomechanical outcome metrics. The results identified 52 eligible DHFP studies, which evaluated various plate and screw variables. The most common plate variables evaluated were geometry, hole type, number, and position. Fewer studies assessed screw variables, with number and angle being the most common. However, no studies examined nonmetallic materials for plates or screws, which may be of interest in future research. Also, articles used various combinations of biomechanical outcome metrics, such as interfragmentary fracture motion, bone, plate, or screw stress, number of loading cycles to failure, and overall stiffness (Os) or failure strength (Fs). However, no study evaluated the bone stress under the plate to examine bone "stress shielding," which may impact bone health clinically. Surgeons treating intraarticular and extraarticular distal humerus fractures should seriously consider two precontoured, long, thick, locked, and parallel plates that are secured by long, thick, and plate-to-plate screws that are located at staggered levels along the proximal parts of the plates, as well as an extra transfracture plate screw. Also, research engineers could improve new studies by perusing recommendations in future work (e.g., studying alternative nonmetallic materials or "stress shielding"), clinical ramifications (e.g., benefits of locked plates), and study quality (e.g., experimental validation of computational studies).

Locking plate constructs in subtrochanteric fixation: a biomechanical comparison of LCP screws and AO-nuts

OBJECTIVES

Various studies have reported the use of the 95-degree condylar blade plate in the treatment of a subtrochanteric fracture or non-union. However, the holding power of standard screws in the metaphyseal and diaphyseal area is often diminished due to osteopenia. The alternative in this area is the use of locking plates, Schühlis or AO-nuts. With the latter two, non-locking screws in the blade plate can be converted to a fixed angle fixation. The objective of this study was to compare the stiffness and strength of the AO-nut augmented 95-degree condylar blade plate construct with that of a locking plate construct. In addition, a clinical series of eight patients treated with the AO-nut augmented 95-degree condylar blade plate construct is presented.

METHODS

Single screw-plate constructs of a 5.0 mm locking screw/locking compression plate (LCP) and a 4.5 mm non-locking screw/4.5 mm dynamic compression plate (DCP), converted to a fixed-angle screw construct using AO-nuts, were tested by cantilever bending. During loading, force and displacement were recorded, from which the bending stiffness (N/mm) and the yield strength (N) were determined. Secondarily, all patients that underwent surgical treatment for subtrochanteric fracture, malunion or non-union by the senior author using this technique, underwent chart review.

RESULTS

The stiffness of the locking screws was about four times higher compared to the AO-nut augmented construct. The yield strength was 2.3 times higher for the locking screw construct. In none of the eight patients treated with the fixed-angle blade plate, failure of the AO-nut augmented construct occurred.

CONCLUSIONS

Although the stiffness and strength of the AO-nut augmented construct is less than of the locking screw, excellent clinical outcomes can be achieved utilizing this construct.

Use of screw locking elements improves radiological and biomechanical results of femoral osteotomies

BACKGROUND

Dynamic compression plate (DCP) constructs provide inadequate fixation in cases of poor bone quality and early weight-bearing. Screw locking elements (SLE) are flat locking nuts placed at the end of the screw to prevent screw stripping from the bone, improving fixation stability. The purpose of this work was to compare biomechanical and radiological evaluations of femoral ovine osteotomies fixed using DCP constructs with and without SLE.

METHOD

A dyaphyseal femoral osteotomy was performed in sixteen adult sheep and fixed with a DCP and cortical screws. Half of the animals were operated on with a SLE on each side of the osteotomy and the rest without the addition of SLE. Four animals of each group were euthanized after 8 weeks, and the remaining after 16 weeks. Both femora of each animal were radiographed and mechanically tested in torsion.

RESULTS

Radiologically femoral malalignment or screw loosening was observed in six out of the eight animals operated on without SLE. In contrast, all animals subjected to the operation with SLE showed complete radiological consolidation of the osteotomy. Seven of these eight animals showed normal femoral alignment and no osteosynthesis failure. Stiffness of the bones fixed with SLE was among 145% and 177% the value of their contralateral non-operated femurs (all animals of this group showed greater stiffness on the operated bone than its contralateral non-operated femur). However, stiffness of the bones operated on without SLE was among 58% and 87% the value of the stiffness of their contralateral non-operated bone (all animals of this group showed greater stiffness on the non-operated bone than the osteotomized ones).

CONCLUSIONS

Use of SLE avoided loosening of the system and stimulated stronger osteotomy consolidation. Clinical application of this improved system may thus be a feasible and cost-effective alternative to other more rigid and expensive bone fixation techniques.

Computational modelling of long bone fractures fixed with locking plates - How can the risk of implant failure be reduced?

BACKGROUND AND PURPOSE

The Locking Compression Plate (LCP) is part of a new plate generation requiring an adapted surgical technique and new thinking about commonly used concepts of internal fixation using plates. Knowledge of the fixation stability provided by these new plates is very limited and clarification is still necessary to determine how the mechanical stability and the risk of implant failure can best be controlled.

METHODS

Upon validation, a finite element model of an LCP attached to a cylinder was developed to simulate and analyse the biomechanics of a transverse long bone fracture fixed with a locking plate. Of special interest were the factors influencing the mechanical conditions at the fracture site, the control of interfragmentary movement and implant failure.

RESULTS

Several factors were shown to influence stability in compression. Increasing translation and/or fracture angle post fixation reduced construct stability. Axial stiffness was also influenced by the working length and plate-bone distance. The fracture gap had no effect on the construct stability when no bone contact occurred during loading. Stress analysis of the LCP demonstrated that the maximum Von Mises stresses were found in the innermost screws at the screw-head junction.

INTERPRETATION

For the clinical use of the LCP as a locked internal fixator in fractures with an interfragmentary gap of 1 mm, at least two to four plate holes near the fracture gap should be omitted to allow fracture motion and bone contact to occur. This will also achieve a larger area of stress distribution on the plate and reduce the likelihood of fatigue failure due to cyclic loading.

Crutch weightbearing on comminuted humeral shaft fractures: a biomechanical comparison of large versus small fragment fixation for humeral shaft fractures

PURPOSE

This study evaluated the failure properties of length unstable humerii secured with small or large fragment plates.

METHODS

Two nonlocking plate constructs were examined, a nine-hole 4.5-mm limited contact dynamic compression plate (large fragment group) and a 12-hole 3.5-mm limited contact dynamic compression plate (small fragment group), both on composite humerii with a 1-cm defect to simulate comminution (n = 12 for each group). Each plate construct had similar working lengths and number of fixation points. Mechanical testing was first randomized for stiffness measurements in axial and torsional loads. All constructs were then tested in cyclic axial loads to failure.

RESULTS

For axial testing, the large fragment group had a mean stiffness of 1020 ± 195 N/mm compared with 268 ± 67 N/mm in the small fragment group (P < 0.0001). For torsional testing, the large fragment group had a mean stiffness of 1.5 ± 0.05 Nm/degree compared with 0.9 ± 0.04 Nm/degree in the small fragment group (P < 0.0001). Plastic deformation in the large fragment and small fragment groups were 0.09 ± 0.07 mm and 0.20 ± 0.24 mm, respectively (P = 0.1) assessed during cyclic testing up to 300 N. The postcyclic yield force in the large fragment group was 227 ± 30 N and in the small fragment group was 153 ± 5 N (P < 0.0001). The ultimate load in the large fragment and small fragment groups were 800 ± 87 N and 307 ± 15 N, respectively.

CONCLUSION

The results corroborate anticipated plate mechanical behavior with plate stiffness increasing as both plate width and thickness increase. The calculated yield force data suggest that both small and large fragment constructs would experience plastic deformation during bilateral crutch ambulation in a patient weighing 50 kg or more. The large fragment construct is not expected to catastrophically fail when subjected to loads in a patient 90 kg or less. The small fragment construct is predicted to catastrophically fail in patients weighing 70 kg or more.

The role of locking technology in the hand

Locked fixed-angle plating in the hand and wrist helps to optimize outcomes following surgical fixation of select acute fractures and complex reconstructions. Select indications include unstable distal ulna head/neck fractures, periarticular metacarpal and phalangeal fractures, comminuted/multifragmentary diaphyseal fractures with bone loss (ie, combined injuries of the hand), osteopenic/pathologic fractures, nonunions and corrective osteotomy fixation, and small joint arthrodesis. Locked plating techniques in the hand should not be seen as a panacea for wrist and digital acute trauma and delayed reconstructions. An understanding of the biomechanics of fixed-angle plating and proper technical application of locking constructs will optimize outcomes and minimize complications. As clinical experience with locking technology in hand trauma broadens, new indications and applications will emerge. Currently, several systems are available. The specific implants share common features in their protocols for insertion, but unique differences in their design (ie, individual locking mechanisms, uniaxial vs polyaxial locking capability, metallurgy, and plate profiles) must be appreciated and considered preoperatively.

A mechanical comparison of the locking compression plate (LCP) and the low contact-dynamic compression plate (DCP) in an osteoporotic bone model

OBJECTIVE

To determine if locking compression plates (LCP) are mechanically advantageous compared to low-contact dynamic compression plates (DCP) when used as a bridging plate in a synthetic model of osteoporotic bone.

METHODS

Five synthetic bars (Synbone Osteoporotic bone) were initially tested in compression and the Young's modulus determined. It was found to be comparable to that of tibial cancellous bone in an 80-year-old woman; thus, the synthetic bars were deemed usable to simulate some properties of osteoporotic bone. Six bars were then instrumented with an 8-hole narrow large fragment DCP with six 4.5-mm cortical screws (placed in holes 1-3 and holes 6-8). Six bars were instrumented with a narrow 8-hole LCP using four 5-mm locking screws (placed in the 1st, 3rd, 6th, and 8th holes). In a third group, 6 synbone bars were instrumented with a narrow 8-hole large fragment DCP. Cortical screws were placed in holes 2-3 and holes 6-7. In holes 1 and 8, two 6.5 fully threaded cancellous screws were inserted. A 1-cm osteotomy was created in the Synbone at the center of each plate to represent a comminuted fracture. Initially, quasi-static testing was carried out on all specimens in compression to a maximum load of 450 N. Then 4-point bend tests were carried out in two planes (0 degrees and 90 degrees) with the maximum bending moment at 3.5 Nm. Finally, torsional testing was done to a maximum load of 3.5 Nm. The specimens were then cycled in axial compression 350 N at 5 Hz for 30,000 cycles. The static nondestructive tests were repeated. The slope of the load deformation curve indicated the relative stiffness of the construct. The slopes were determined pre- and postcycling and the loss of stiffness in each group compared. Statistical analysis was carried out using the paired t-test. The specimens were then loaded to failure in compression.

RESULTS

There was no statistical difference in the stiffness of the LCP or in the osteotomy gap postcycling. All specimens in the DCP groups failed initial static testing in axial compression. No fatigue testing could be undertaken in this group.

CONCLUSIONS

In a synthetic model, the LCP was mechanically superior to the DCP when used as a bridging plate and tested in axial compression.

Biomechanical analysis of plate osteosynthesis systems for proximal humerus fractures

OBJECTIVES

To biomechanically assess five proximal, humeral, fracture-plate-fixation systems.

METHODS

Surgical neck fractures, with and without cortical contact, were created in 25 fresh-frozen cadaveric humeri. Five methods of plate fixation were used for repair: construct A [an eight-hole, low contact dynamic compression (LCDC) plate contoured into a blade shape, supported by one, 70-mm-long, 4.5-mm-diameter cortical screw acting as a truss], construct B (a 10-hole LCDC plate arrangement identical to construct A, but using one, 70-mm-long, 3.5-mm-diameter cortical screw as a truss), construct C [a five-hole dynamic compression (DC) blade plate with one, 6.5-mm-diameter cancellous screw], construct D (a five-hole T-plate supported by three, 6.5-mm-diameter cancellous screws), and construct E (a five-hole cloverleaf plate supported by five, 4-mm-diameter cancellous screws). Plates were posterior to the bicipital groove, 10 mm distal to the greater tuberosity tip, on the lateral aspect of the humeral shaft. Screw fixation was done using standard AO compression plating techniques. Stiffness of constructs was measured in bending and axial compression. Locked plates were not assessed.

RESULTS

For cortical contact [abduction of 20 degrees (P=0.02), flexion of 20 degrees (P=0.02), flexion of 90 degrees (P=0.005)] and no cortical contact [flexion of 90 degrees (P=0.0001)], construct A was significantly stiffer than other constructs. For no cortical contact in abduction of 90 degrees (P=0.05), construct A was significantly stiffer than other constructs.

CONCLUSIONS

Construct A was significantly stiffer than other constructs.

Principles of fixation of osteoporotic fractures

Despite advances in the prevention and treatment of osteoporotic fractures, their prevalence continues to increase. Their operative treatment remains a challenge for the surgeon, often with unpredictable outcomes. This review highlights the current aspects of management of these fractures and focuses on advances in implant design and surgical technique.

The mechanical behavior of locking compression plates compared with dynamic compression plates in a cadaver radius model

OBJECTIVE

The purpose of this cadaveric study was to compare the mechanical behavior of a locked compression plate, which uses threaded screw heads to create a fixed angle construct, with a dynamic compression plate construct in a cadaver radius model.

DESIGN

Mechanical study with cyclic testing and high-speed optical motion analysis.

SETTING

Biomechanics laboratory at an academic institution.

PATIENTS/PARTICIPANTS

Eighteen pairs of fresh-frozen human cadaver radii were divided into 3 groups of 6 to be tested as a group in each of the following force applications: anteroposterior (AP) bending, mediolateral bending, or torsion.

INTERVENTION

Each bone was osteotomized leaving a 5-mm fracture gap and then fixed with a plate. For each pair, 1 radius received a standard plate (limited-contact dynamic compression plates; LC-DCP), the contralateral radius was fixed with a locking compression plate (LCP), and specimens underwent cyclic loading. Normalized stiffness, average energy absorbed, and Newton-cycles to failure were calculated. In addition, a 3-dimensional, high-speed, infrared motion analysis system was used to evaluate motion at the fracture site.

MAIN OUTCOME MEASUREMENTS

Construct stiffness, fracture site motion, cycles to failure, and energy absorption. Repeated measures ANOVA were used to detect differences between groups with time.

RESULTS

In the torsion group, LCP specimens failed at 60% greater Newton-cycles than the LC-DCP (1473 vs. 918; P < 0.05). In the AP group, the LC-DCP absorbed significantly greater energy during 10,000 cycles compared with the LCP group (P < 0.05). The 2 constructs demonstrated different biomechanical behavior with time. As cycling progressed in the LC-DCP specimens under torsion testing, stiffness (measured at the actuator at the bone ends) did not change significantly; however, fracture motion (measured at the fracture surfaces) decreased significantly (P = 0.04). The LCP specimens did not display similar behavior.

CONCLUSIONS

Our findings indicated that LCP constructs may demonstrate subtle mechanical superiority compared with the LC-DCP. The LCP specimens had less energy absorption in the AP group and survived longer in the torsion group. Discordance of motion between measurement regions was observed only in the LC-DCP torsion group, and may have been caused by plate-bone slippage or bone-screw subcatastrophic failure. However, many other compared parameters were found to be similar, and the clinical significance of the few differences found between constructs mandates further investigation.

Locking compression plates for osteoporotic nonunions of the diaphyseal humerus

Poor bone quality increases the technical difficulty and complications of operative treatment of nonunions and delayed unions of the diaphyseal humerus in older patients. Plates with screws that lock to the plate (transforming each screw into a fixed blade) are intended to improve the fixation of poor quality bone. Twenty-four patients (20 women, four men) with an average age of 72 years (range, 52-86 years) were followed up for a minimum of 12 months after locking compression plate fixation of an osteopenic delayed union (nine patients) or nonunion (15 patients) of the diaphyseal humerus. Twelve patients had iliac crest cancellous bone grafts, two patients had local graft, and 13 patients had demineralized bone applied to the fracture site. All the fractures eventually healed; two healed after a second procedure for autogenous bone grafting in patients who initially received demineralized bone. Using a modification of the Constant and Murley shoulder score, the results were good or excellent in 22 patients, and fair in two patients. Locking compression plates provide stable fixation of poor quality bone in patients with delayed union or nonunion of the humerus. Successful union and restoration of function are achieved in most patients. We no longer consider osteoporosis a contraindication to operative fixation of an ununited fracture of the humeral diaphysis.

Ununited diaphyseal fractures of the humerus: techniques for fixation of osteoporotic bone

The elderly population is larger, healthier, and more active than ever. Osteoporosis-related fractures and their sequelae are becoming more commonplace. Successful management of these problems requires adequate internal fixation of osteoporotic bone. Fixation of osteoporotic bone has evolved from the use of cement or allograft-augmented screws to fixed-angle plates, and now to fixed-angle screws that thread into and engage solidly the hole in the plate, creating a fixed blade. Familiarity with these techniques may facilitate the care of elderly patients requiring operative fracture care.

Design and validation of a surrogate humerus for biomechanical testing

At present biomechanical testing of fracture plating strategies is conducted using animal or cadaveric whole bone models. This may introduce experimental error into these studies. This communication summarises the design and validation of a novel bone and fibre-reinforced plastic construct conceived to minimise intra-experimental error. A tubular surrogate humerus was produced with dimension and strength matched to that of the human humerus. Bone inserts placed into the wall of the tube allow for the fixation of the plates with bone screws. Three-point bending tests of the flexural rigidity of the surrogate humerus (EI=100.1 (SD 6.0)Nm(2)) showed it to be comparable to the human humerus. Further, pull-out tests of the screws showed that the bone slots adequately mimicked the whole bone scenario. This testing construct will be used for a comparative study of humeral plating techniques.