The biomechanics of locked plating for repairing proximal humerus fractures with or without medial cortical support

Open reduction internal fixation of proximal humerus fractures

The treatment of proximal humerus fractures continues to evolve. While the many of these injuries can be managed nonoperatively, a certain percentage require operative treatment. Open reduction internal fixation can offer excellent outcomes when performed in the appropriate patient and utilizing proper techniques. This article reviews the most up-to-date literature regarding all phases of proximal humerus fracture osteosynthesis, including diagnosis, imaging, anatomic considerations, surgical indications, fixation, and surgical outcomes.

Locking plate and fibular allograft augmentation in unstable fractures of proximal humerus

INTRODUCTION

The use of an intramedullary fibular allograft together with a locking plate fixation has been recently described in biomechanical studies to provide additional medial support and prevent varus malalignment for displaced proximal humeral fractures with promising results. The aim of this study was to evaluate the clinical and radiographic outcome of a locking plate with fibular allograft augmentation in unstable humeral fractures.

METHODS

We prospectively assessed the functional outcome and complications in 17 patients with proximal humeral fractures with disrupted medial column, treated with a locking plate and a fibular strut graft. The median patient age was 62 years. Postoperative assessments included radiographic imaging, range of motion, pain according to the visual analogue scale (VAS), Short Form (SF36) Health Survey, Constant-Murley and Disabilities of the Arm, Shoulder and Hand (DASH) shoulder scores as well as return to previous occupation and complications.

RESULTS

No patients were lost to follow-up and no major complications were recorded. There was no collapse of the humeral head more than 2mm, osteonecrosis or screw penetration of the articular surface. All fractures healed clinically and radiographically. After an average follow-up of 13 months, the mean Constant score was 79 points. The mean active flexion was 149°; extension, 47°; internal rotation, 40°; external rotation, 65°; and abduction, 135°. The median VAS pain level was 1 point. The median DASH score was 33 points, and the median SF36 was 83 points.

CONCLUSION

Locking plate with fibular graft augmentation is a safe and promising technique to support the humeral head and maintain reduction in the proximal humeral fracture with medial comminution.

Calcar comminution as prognostic factor of clinical outcome after locking plate fixation of proximal humeral fractures

OBJECTIVE

In the treatment of proximal humeral fractures, the decision between open fixation and arthroplasty is often difficult. Applicable radiographic prognostic factors would be useful. The purpose of the present study was to investigate the influence of calcar comminution on the clinical and radiologic outcome after locking plate fixation of these fractures.

METHODS

In patients with proximal humeral fractures that were treated by locking plate fixation, fracture morphology and the presence of comminution of the calcar were documented on preoperative radiographs. Follow-up for at least 2 years with radiologic assessment and functional outcome measurements including Constant score, subjective shoulder value (SSV), disabilities of the arm, shoulder and hand score (DASH), visual analogue scale (VAS) and short form (SF)-36 was performed.

RESULTS

Follow-up examination (50.8±20.6 months) was possible in 74 patients (46 female, 28 male, age 63.0±15.9 years). Mean absolute Constant score (CS abs), CS adapted to age and gender (CS adap), DASH, SSV and VAS were 72.4±14.5, 85.2±17.3%, 15.7±17.3, 80.3±19.6% and 2.1±2.2. Nonunion was present in 1.3%, cut-out in 5.4% and implant failure in 1.3%. Avascular necrosis (AVN) was seen in 12.2%, in three cases >24 months after the initial trauma. In the presence of calcar comminution, the clinical outcome (CS abs, CS adap, SSV and several parameters of SF-36) was significantly impaired, the odds ratio for these patients to have an absolute CS<65 was 4.4 (95% confidence interval (CI): 1.4-13.7).

CONCLUSIONS

The treatment of proximal humeral fractures with locking plate fixation achieves good clinical mid-term results. Calcar comminution is a relevant and easy-to-detect prognostic factor for the functional and subjective outcome in these fractures.

The biomechanical effect of torsion on humeral shaft repair techniques for completed pathological fractures

In the presence of a tumor defect, completed humeral shaft fractures continue to be a major surgical challenge since there is no "gold standard" treatment. This is due, in part, to the fact that only one prior biomechanical study exists on the matter, but which only compared 2 repair methods. The current authors measured the humeral torsional performance of 5 fixation constructs for completed pathological fractures. In 40 artificial humeri, a 2-cm hemi-cylindrical cortical defect with a transverse fracture was created in the lateral cortex. Specimens were divided into 5 different constructs and tested in torsion. Construct A was a broad 10-hole 4.5-mm dynamic compression plate (DCP). Construct B was the same as A except that the screw holes and the tumor defect were filled with bone cement and the screws were inserted into soft cement. Construct C was the same as A except that the canal and tumor defect were filled with bone cement and the screws were inserted into dry cement. Construct D was a locked intramedullary nail inserted in the antegrade direction. Construct E was the same as D except that bone cement filled the defect. For torsional stiffness, construct C (4.45 ± 0.20 Nm/deg) was not different than B or E (p > 0.16), but was higher than A and D (p < 0.001). For failure torque, construct C achieved a higher failure torque (69.65 ± 5.35 Nm) than other groups (p < 0.001). For the failure angle, there were no differences between plating constructs A to C (p ≥ 0.11), except for B versus C (p < 0.05), or between nailing groups D versus E (p = 0.97), however, all plating groups had smaller failure angles than both nailing groups (p < 0.05). For failure energy, construct C (17.97 ± 3.59 J) had a higher value than other groups (p < 0.005), except for A (p = 0.057). Torsional failure always occurred in the bone in the classic "spiral" pattern. Construct C provided the highest torsional stability for a completed pathological humeral shaft fracture.

A preliminary biomechanical study of cyclic preconditioning effects on canine cadaveric whole femurs

Biomechanical preconditioning of biological specimens by cyclic loading is routinely done presumably to stabilize properties prior to the main phase of a study. However, no prior studies have actually measured these effects for whole bone of any kind. The aim of this study, therefore, was to quantify these effects for whole bones. Fourteen matched pairs of fresh-frozen intact cadaveric canine femurs were sinusoidally loaded in 4-point bending from 50 N to 300 N at 1 Hz for 25 cycles. All femurs were tested in both anteroposterior (AP) and mediolateral (ML) bending planes. Bending stiffness (i.e., slope of the force-vs-displacement curve) and linearity R(2) (i.e., coefficient of determination) of each loading cycle were measured and compared statistically to determine the effect of limb side, cycle number, and bending plane. Stiffnesses rose from 809.7 to 867.7 N/mm (AP, left), 847.3 to 915.6 N/mm (AP, right), 829.2 to 892.5 N/mm (AP, combined), 538.7 to 580.4 N/mm (ML, left), 568.9 to 613.8 N/mm (ML, right), and 553.8 to 597.1 N/mm (ML, combined). Linearity R(2) rose from 0.96 to 0.99 (AP, left), 0.97 to 0.99 (AP, right), 0.96 to 0.99 (AP, combined), 0.95 to 0.98 (ML, left), 0.94 to 0.98 (ML, right), and 0.95 to 0.98 (ML, combined). Stiffness and linearity R(2) versus cycle number were well-described by exponential curves whose values leveled off, respectively, starting at 12 and 5 cycles. For stiffness, there were no statistical differences for left versus right femurs (p = 0.166), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p < 0.0001). Similarly, for linearity, no statistical differences were noted due to limb side (p = 0.533), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p = 0.006). A minimum of 12 preconditioning cycles was needed to fully stabilize both the stiffness and linearity of the canine femurs. This is the first study to measure the effects of mechanical preconditioning on whole bones, having some practical implications on research practices.

Biomechanical measurements of cortical screw stripping torque in human versus artificial femurs

Femur fracture plates are applied using cortical bone screws. Surgeons do this manually by subjective ‘feel’ without monitoring torque. Few studies have quantified stripping torque in human bone. No studies have measured stripping torque in the artificial bones from Sawbones (Vashon, WA, USA) that are frequently used in biomechanical studies. The present aim was to measure stripping torque of cortical screws in human versus artificial femurs. Sixteen fresh-frozen human femurs and eight artificial femurs were used. Using a digital torque screwdriver, each femur had a 3.5-mm diameter unicortical screw manually inserted into the anterior midshaft until failure of the screw–bone interface. Results were normalized by cortical thickness and the screw–bone interfacial area. There were no statistical differences in human versus artificial data, respectively, for stripping torque (1741 ± 442 N.mm, 2012 ± 176 N.mm, p = 0.11), stripping torque/thickness (313 ± 59 N, 305 ± 30 N, p = 0.74), and stripping torque/area (28.5 ± 5.3 N/mm, 27.8 ± 2.8 N/mm, p = 0.74). Artificial unicortical thickness (6.6 ± 0.3 mm) was greater than human thickness (5.6 ± 1.1 mm) ( p = 0.02). For human specimens, there was a moderate linear correlation of absolute and normalized stripping torque versus standardized bone mineral density (R ≥ 0.32) and clinical T-score (R = 0.29), but not with age (R ≤ 0.29). Surgeons should be aware of the stripping torque limits for human femurs and potentially take steps to monitor these values during surgery. The artificial femurs being increasingly used in research accurately replicate human cortical properties during screw insertion. To date, this is the first series of human femurs evaluated for cortical screw stripping.

Biomechanical measurements of axial crush injury to the distal condyles of human and synthetic femurs

Few studies have evaluated the ‘bulk’ mechanical properties of human longbones and even fewer have compared human tissue to the synthetic longbones increasingly being used by researchers. Distal femur fractures, for example, comprise about 6% of all femur fractures, but the mechanical properties of the distal condyles of intact human and synthetic femurs have not been well quantified in the literature. To this end, the distal portions of a series of 16 human fresh-frozen femurs and six synthetic femurs were prepared identically for mechanical testing. Using a flat metal plate, an axial ‘crush’ force was applied in-line with the long axis of the femurs. The two femur groups were statistically compared and values correlated to age, size, and bone quality. Results yielded the following: crush stiffness (human, 1545 ± 728 N/mm; synthetic, 3063 ± 1243 N/mm; p = 0.002); crush strength (human, 10.3 ± 3.1 kN; synthetic, 12.9 ± 1.7 kN; p = 0.074); crush displacement (human, 6.1 ± 1.8 mm; synthetic, 2.8 ± 0.3 mm; p = 0.000); and crush energy (human, 34.8 ± 15.9 J; synthetic, 18.1 ± 5.7 J; p = 0.023). For the human femurs, there were poor correlations between mechanical properties versus age, size, and bone quality (R2 ≤ 0.18), with the exception of crush strength versus bone mineral density (R2 = 0.33) and T-score (R2 = 0.25). Human femurs failed mostly by condyle ‘roll back’ buckling (15 of 16 cases) and/or unicondylar or bicondylar fracture (7 of 16 cases), while synthetic femurs all failed by wedging apart of the condyles resulting in either fully or partially displaced condylar fractures (6 of 6 cases). These findings have practical implications on the use of a flat plate load applicator to reproduce real-life clinical failure modes of human femurs and the appropriate use of synthetic femurs. To the authors’ knowledge, this is the first study to have done such an assessment on human and synthetic femurs.

The calcar screw in angular stable plate fixation of proximal humeral fractures--a case study

BACKGROUND

With new minimally-invasive approaches for angular stable plate fixation of proximal humeral fractures, the need for the placement of oblique inferomedial screws ('calcar screw') has increasingly been discussed. The purpose of this study was to investigate the influence of calcar screws on secondary loss of reduction and on the occurrence of complications.

METHODS

Patients with a proximal humeral fracture who underwent angular stable plate fixation between 01/2007 and 07/2009 were included. On AP views of the shoulder, the difference in height between humeral head and the proximal end of the plate were determined postoperatively and at follow-up. Additionally, the occurrence of complications was documented. Patients with calcar screws were assigned to group C+, patients without to group C-.

RESULTS

Follow-up was possible in 60 patients (C+ 6.7 ± 5.6 M/C- 5.0 ± 2.8 M). Humeral head necrosis occurred in 6 (C+, 15.4%) and 3 (C-, 14.3%) cases. Cut-out of the proximal screws was observed in 3 (C+, 7.7%) and 1 (C-, 4.8%) cases. In each group, 1 patient showed delayed union. Implant failure or lesions of the axillary nerve were not observed. In 44 patients, true AP and Neer views were available to measure the head-plate distance. There was a significant loss of reduction in group C- (2.56 ± 2.65 mm) compared to C+ (0.77 ± 1.44 mm; p = 0.01).

CONCLUSIONS

The placement of calcar screws in the angular stable plate fixation of proximal humeral fractures is associated with less secondary loss of reduction by providing inferomedial support. An increased risk for complications could not be shown.

Medial support by fibula bone graft in angular stable plate fixation of proximal humeral fractures: an in vitro study with synthetic bone

BACKGROUND

Failure to achieve stable fixation with medial support in proximal humeral fractures can result in varus malalignment and cut-through of the proximal screws. The purpose of this study was to investigate the influence of an intramedullary fibula bone graft on the biomechanical properties of proximal humeral fractures stabilized by angular stable plate fixation in a bone model under cyclic loading.

METHODS

Two fixation techniques were tested in 20 composite analog humeri models. In group F- (n = 10), fractures were fixed by an anatomically formed locking plate system. In group F+ (n = 10), the same fixation system was used with an additional fibular graft model with a length of 6 cm inserted in an intramedullary manner. Active abduction was simulated for 400 cycles by use of a recently established testing setup. Fragment gap distance was measured, and thereby, intercyclic motion, fragment migration, and residual plastic deformation were determined.

RESULTS

The addition of a fibular graft to the fixation plate led to 5 times lower intercyclic motion, 2 times lower fragment migration, and 2 times less residual plastic deformation. Neither screw pullout, cut-through, nor implant failure was observed.

CONCLUSION

Medial support with an intramedullary fibular graft in an angular stable fixation of the proximal humerus in vitro increases overall stiffness of the bone-implant construct and reduces migration of the humeral head fragment. This technique might provide a useful tool in the treatment of displaced proximal humeral fractures, especially when there is medial comminution.

The clinical benefit of medial support screws in locking plating of proximal humerus fractures: a prospective randomized study

PURPOSE

The purpose of this study was to evaluate the clinical benefit of medial support screws for locking plating of proximal humerus fractures.

METHODS

Seventy-two consecutive patients underwent prospective treatment for proximal humerus fractures with locking plates between October 2007 and September 2008. Sixty-eight patients accomplished a mean 30.8-month follow-up and were randomized into two groups: 39 patients were treated with only a locking plate and were classified in the -MSS (medial support screw) group, and 29 patients were included in the + MSS group, which were fixed with additional medial support screws. Clinical and radiological investigations were performed in both groups.

RESULTS

The fractures united at an average of 13.6 weeks after final surgery. Comparably better shoulder function recovery was achieved in the +MSS group with regard to the Constant shoulder score (P = 0.01), with the respective excellent and good rates of 79% and 62%. Eleven patients developed various complications. A statistical difference (P = 0.036) was observed regarding the failure rate (23.1% in the -MSS group vs. 3.4% in the +MSS group). The early loss of fixation was related to higher age (P < 0.001) and less initial neck-shaft angle (NSA) (P = 0.011) of the patients. However, bone mineral density was not significantly associated with loss of fixation (P = 0.076). Although no difference was found in all types of the fractures between the +MSS and -MSS groups regarding immediate postoperative NSA, we observed a significantly lower final NSA in the -MSS group and greater secondary angle loss in the subgroup of Neer three-part (P = 0.033 and 0.015, respectively) and four-part fractures (P = 0.043 and 0.027).

CONCLUSIONS

Anatomical reduction can substantially decrease the risk of postoperative failure in locking plating of proximal humerus fractures. Medial support for proximal humerus fractures seems to have no benefits in Neer two-part fractures. However, the additional medial support screws inserted into the medio-inferior region of the humeral head may help to enhance mechanical stability in complex fractures and allow for better maintenance of reduction.