Dirk Wähnert

Internal Fixation of Type-C Distal Femoral Fractures in Osteoporotic Bone

BACKGROUND Fixation of distal femoral fractures remains a challenge, especially in osteoporotic bone. This study was performed to investigate the biomechanical stability of four different fixation devices for the treatment of comminuted distal femoral fractures in osteoporotic bone. METHODS Four fixation devices were investigated biomechanically under torsional and axial loading. Three intramedullary nails, differing in the mechanism of distal locking (with two lateral-to-medial screws in one construct, one screw and one spiral blade in another construct, and four screws [two oblique and two lateral-to-medial with medial nuts] in the third), and one angular stable plate were used. All constructs were tested in an osteoporotic synthetic bone model of an AO/ASIF type 33-C2 fracture. Two nail constructs (the one-screw and spiral blade construct and the four-screw construct) were also compared under axial loading in eight pairs of fresh-frozen human cadaveric femora. RESULTS The angular stable plate constructs had significantly higher torsional stiffness than the other constructs; the intramedullary nail with four-screw distal locking achieved nearly comparable results. Furthermore, the four-screw distal locking construct had the greatest torsional strength. Axial stiffness was also the highest for the four-screw distal locking device; the lowest values were achieved with the angular stable plate. The ranking of the constructs for axial cycles to failure was the four-screw locking construct, with the highest number of cycles, followed by the angular stable plate, the spiral blade construct, and two-screw fixation. The findings in the human cadaveric bone were comparable with those in the synthetic bone model. Failure modes under cyclic axial load were comparable for the synthetic and human bone models. CONCLUSIONS The findings of this study support the concept that, for intramedullary nails, the kind of distal interlocking pattern affects the stabilization of distal femoral fractures. Four-screw distal locking provides the highest axial stability and nearly comparable torsional stability to that of the angular stable plate; the four-screw distal interlocking construct was found to have the best combined (torsional and axial) biomechanical stability.

Distal femur fractures of the elderly--different treatment options in a biomechanical comparison.

BACKGROUND Fractures of the distal femur, especially in the elderly patient, are an unsolved problem in orthopaedic and trauma surgery. Poor bone stock quality caused by osteoporosis often results in bad implant anchorage in the distal part with a high risk of secondary failures such as cutout. This study investigates the biomechanical characteristics of four implants with different distal locking options under quasi-static torsional and cyclic axial loading. Therefore, an osteoporotic bone model simulating severe osteoporotic conditions was used. METHODS Four different implants (T2 intramedullary nail, supracondylar nail (SCN), distal femoral nail (DFN) and the AxSOS angular stable plate) with different distal locking options were instrumented using an osteoporotic bone model. Five specimens per implant and per loading type (torsional and axial) were used. Mechanical testing was performed under physiologic loading conditions. First, a torsional test was performed in internal and external rotation (10 Nm), with a new specimen; a stepwise cyclic axial loading was conducted until failure of the construct. FINDINGS For torsional loading, the lowest range of motion (ROM) and neutral zone (NZ) was found for the AxSOS plate construct. The SCN and T2 constructs showed similar results, and the highest ROM and NZ were found for the constructs treated with the DFN. Axial stiffness was highest for SCN constructs and in the same range for DFN and T2. The lowest stiffness showed in the AxSOS plate constructs with 47% of SCN stiffness. Under cyclic axial loading, the SCN constructs showed the highest number of cycles to failure, followed by AxSOS (70%), DFN (69%) and T2 (48%). INTERPRETATION In conclusion of this biomechanical study, we can clinically suggest that, if, in general, torsional stability is required (e.g., for bedridden patients) the AxSOS plate will be sufficient. By contrast, the findings of this study support the fact that the SCN should be considered for mobile patients where early postoperative mobilisation for rehabilitation is desired.

Cement augmentation of the navigated iliosacral screw in the treatment of insufficiency fractures of the sacrum: a new method using modified implants.

PurposePelvis ring fractures of geriatric patients are currently a serious problem in orthopedic and trauma surgery. One controversy that remains is the insufficiency fracture of the sacrum with treatment options ranging from conservative to operative. We modified standard 7.3-mm screws with additional perforations at the tip to allow cement injection trough the screw to improve implant anchorage.MethodWe describe a new method of treatment of sacral insufficiency fractures. We use modified cannulated 7.3-mm screws (Synthes) with additional perforations at the screw tip. This allows the implant augmentation after proper implant positioning and contrast medium instillation for leakage detection.ResultsAll patients (12 female) treated this way, are allowed to weight bear as tolerated immediately after surgery. The pain level (measured by the visual analog scale—VAS) was significantly reduced due to surgery (mean 8.2 pre-operatively, mean 2.6 postoperatively), the pain medication could be reduced rapidly.ConclusionThe described procedure for the treatment of sacral insufficiency fractures has the potential to increase the safety of cement injection into the sacrum because of the possibility of contrast agent instillation prior to cement injection. Furthermore, the amount of cement can be reduced and additional stability can be attained due to iliosacral screw osteosynthesis compared to sacroplasty.

Biomechanical comparison of two angular stable plate constructions for periprosthetic femur fracture fixation

PurposeFractures of the femur associated with total hip arthroplasty are a significant concern in orthopaedic and trauma surgery. However, little is known about the different biomechanical properties of internal fixation systems in combination with periprosthetic fractures. In this study two new internal fixation systems for periprosthetic fractures are investigated using a cadaver fracture model simulating a Vancouver B1 periprosthetic femur fracture.MethodsNine pairs of fresh-frozen cadaver femurs were scanned by dual X-ray absorptiometry. Cementless total hip prostheses were implanted and a periprosthetic femur fracture was simulated. Fractures were randomly fixed either with the fixed angle locking attachment plate (LAP®, Depuy Synthes®, Solothurn, Switzerland) or the variable angle non-contact bridging plate (NCB®, Zimmer GmbH, Winterthur, Switzerland). Each construct was cyclically loaded to failure in axial compression.ResultsAxial stiffness and cycles to failure were significantly higher in the NCB group. Both systems were able to be fixed well around the femoral stem.ConclusionThe two different internal fixation systems for periprosthetic fractures differed significantly in our setup. The non-contact bridging plate system revealed significantly higher failure load and may be the preferred option where high stability and load capacity is needed right after operation.

Long-term stability of angle-stable versus conventional locked intramedullary nails in distal tibia fractures

BackgroundIn the last years intramedullary nailing has become the treatment of choice for most displaced diaphyseal tibia fractures. In contrast intramedullary nailing of distal tibia fractures is accompanied by problems like decreased biomechanical stability. Nevertheless the indications for intramedullary nailing have been extended to include even more distal fractures. The purpose of this study was to compare long-term mechanical characteristics of angle-stable versus conventional locked intramedullary nails in the treatment of unstable distal tibia fractures. Therefore, the effect of time on the mechanical properties of biodegradable sleeves was assessed.Methods8 pairs of fresh, frozen porcine tibiae were used. The expert tibial nail (Synthes) was equipped with either three conventional locking screws (CL) or the angle-stable locking system (AS), consisting of a special ASLS screw and a biodegradable sleeve. Biomechanical testing included torsional and axial loading at different time-points over 12 weeks.ResultsThe AS group showed a significantly higher torsional stiffness at all time-points (at least 60%) compared to the CL group (p < 0.001). The neutral zone was at least 5 times higher in the CL group (p < 0.001). The mean axial stiffness was maximum 10% higher (week 6) in the angle-stable locked group compared to the conventional group. There was no significant change of the torsional mechanical characteristics over the 12 weeks in both groups (p > 0.05). For axial stiffness and range of motion significant differences were found in the AS group.ConclusionsThe angle-stable locking system (ASLS) with the biodegradable sleeve provides significantly higher long-term stability. Especially the differences determined under torsional loading in this study may have clinical relevance. The ASLS permits the potential to decrease complications like secondary loss of reduction and mal-/non-union.

The potential of implant augmentation in the treatment of osteoporotic distal femur fractures: A biomechanical study

PURPOSE Osteoporotic fractures of the distal femur are an underestimated and increasing problem in trauma and orthopaedic surgery. Therefore, this study investigates the biomechanical potential of implant augmentation in the treatment of these fractures. METHODS Twelve osteoporotic surrogate distal femora were randomly assigned to the augmented or non-augmented group. All specimens were fixed using the LCP DF. In the augmented group additionally 1ml Vertecem V+ was injected in each screw hole before screw positioning. The construct represents an AO 33 A3 fracture. Biomechanical testing was performed as sinusoidal axial loading between 50 and 500N with 2Hz for 45,000 cycles, followed by loading between 50 and 750N until failure. RESULTS The augmented group showed significant higher axial stiffness (36%). Additionally the displacement after 45,000 cycles was 3.4 times lower for the augmented group (0.68±0.2mm vs. 2.28±0.2mm). Failure occurred after 45,130 cycles (SD 99) in all of the non-augmented specimens and in two specimens of the augmented group after 69,675 cycles (SD 1729). Four of the augmented specimens showed no failure. The failure mode of all specimens in both groups was a medial cut-out. CONCLUSIONS This study shows a promising potential of implant augmentation in the treatment of osteoporotic distal femur fractures.

How to enhance the stability of locking plate fixation of proximal humerus fractures? An overview of current biomechanical and clinical data

BACKGROUND The complication rate after locking plate fixation of proximal humerus fractures is high. In addition to low bone mineral density, a lack of medial support has been identified as one of the most important factors accounting for mechanical instability. As a result of the high failure rate, different strategies have been developed to enhance the mechanical stability of locking plate fixation of proximal humerus fractures. The aim of the present article is to give an overview of the current biomechanical and clinical studies that focus on how to increase the stability of locking plate fixation of proximal humerus fractures. METHODS A comprehensive search of the Medline databases using specific search terms with regard to the stability of locking plate fixation of proximal humerus fractures was performed. After screening of the articles for eligibility, they were subdivided according to clinical and biomechanical aspects. RESULTS Medial support screws, filling of bone voids and screw-tip augmentation with bone cement as well as the application of bone grafts are currently the most frequently assessed and performed methods. Although the evidence is weak, all of the mentioned strategies appear to have a positive effect on achieving and maintaining a stable reduction even of complex fractures. CONCLUSION Further clinical studies with a higher number of patients and a higher level of evidence are required to develop a standardised treatment algorithm with regard to cement augmentation and bone grafting. Although these measures are likely to have a stabilising effect on locking plate fixation, its general use cannot be fully recommended yet.

Limited V-shaped cement augmentation of the proximal femur to prevent secondary hip fractures.

Patients with a femoral fracture due to osteoporosis are at high risk of sustaining a secondary fracture on the contralateral side. A prophylactic mechanical reinforcement of the contralateral side during operation of the initial fracture could be of interest for such patients. This biomechanical in vitro study investigates the potential of a limited V-shaped bone cement augmentation to prevent secondary hip fractures by targeting the areas of the proximal femur with the highest stresses during a fall. Five pairs of human cadaveric proximal femora were tested in a configuration simulating a fall on the greater trochanter. The femoral neck of one specimen of each pair was augmented with 8–14 ml polymethylmethacrylate from the lateral cortex towards inferior and superior, spanning a V-shaped cement pattern. Clinical relevant fractures were generated with a 45 kg mass in controlled free fall. Load-displacement data were recorded and energy to fracture, fracture load, yield load and stiffness were statistically evaluated. Augmented samples absorbed 124% more energy until fracture compared to their controls (p = 0.043). No significant differences were found between the two groups for fracture load (p = 0.5), yield load (p = 0.35) and stiffness (p = 0.5). Biomechanically, a limited V-shaped prophylactic cement augmentation carries potential to prevent secondary hip fractures indicated by increased energy absorption until fracture. Further investigations are necessary to minimize interference with the biology and to maximize the mechanical benefit of prophylactic augmentation.

Screw augmentation reduces motion at the bone-implant interface: a biomechanical study of locking plate fixation of proximal humeral fractures

BACKGROUND Shear forces at the bone-implant interface lead to a loss of reduction after locking plate fixation of proximal humeral fractures. The aim of the study was to analyze the roles of medial support screws and screw augmentation in failure loads and motion at the bone-implant interface after locking plate fixation of proximal humeral fractures. METHODS Unstable 3-part fractures were simulated in 6 pairs of cadaveric humeri and were fixed with a DiPhos-H locking plate (Lima Corporate, Udine, Italy). An additional medial support screw was implanted in 1 humerus of every donor. The opposite humerus was stabilized with a medial support screw and additional bone cement augmentation of the 2 anteriorly directed head screws. Specimens were loaded in the varus bending position. Stiffness, failure loads, plate bending, and the motion at the bone-implant interface were evaluated using an optical motion capture system. RESULTS The mean load to failure was 669 N (standard deviation [SD], 117 N) after fixation with medial support screws alone and 706 N (SD, 153 N) after additional head screw augmentation (P = .646). The initial stiffness was 453 N/mm (SD, 4.16 N/mm) and 461 N/mm (SD, 64.3 N/mm), respectively (P = .594). Plate bending did not differ between the 2 groups. However, motion at the bone-implant interface was significantly reduced after head screw augmentation (P < .05). CONCLUSION The addition of bone cement to augment anteriorly directed head screws does not increase stiffness and failure loads but reduces motion at the bone-implant interface. Thus, the risk of secondary dislocation of the head fragment may be reduced.

Implant Augmentation: Adding Bone Cement to Improve the Treatment of Osteoporotic Distal Femur Fractures: A Biomechanical Study Using Human Cadaver Bones

AbstractThe increasing problems in the field of osteoporotic fracture fixation results in specialized implants as well as new operation methods, for example, implant augmentation with bone cement. The aim of this study was to determine the biomechanical impact of augmentation in the treatment of osteoporotic distal femur fractures.Seven pairs of osteoporotic fresh frozen distal femora were randomly assigned to either an augmented or nonaugmented group. In both groups, an Orthopaedic Trauma Association 33 A3 fractures was fixed using the locking compression plate distal femur and cannulated and perforated screws. In the augmented group, additionally, 1 mL of polymethylmethacrylate cement was injected through the screw. Prior to mechanical testing, bone mineral density (BMD) and local bone strength were determined. Mechanical testing was performed by cyclic axial loading (100 N to 750 N + 0.05N/cycle) using a servo-hydraulic testing machine.As a result, the BMD as well as the axial stiffness did not significantly differ between the groups. The number of cycles to failure was significantly higher in the augmented group with the BMD as a significant covariate.In conclusion, cement augmentation can significantly improve implant anchorage in plating of osteoporotic distal femur fractures.