Kay Sellenschloh

Biomechanical Evaluation of 3 Stabilization Methods on Acromioclavicular Joint Dislocations

Background: Traumatic acromioclavicular (AC) joint dislocations can be addressed with several surgical stabilization techniques. The aim of this in vitro study was to evaluate biomechanical features of the native joint compared with 3 different stabilization methods: locking hook plate (HP), TightRope (TR), and bone anchor system (AS). Hypothesis: The HP provides higher stiffness than the anatomic reconstruction techniques. Study Design: Controlled laboratory study. Methods: A new biomechanical in vitro model of the AC joint was used to analyze joint stability after surgical repair (HP, TR, and AS). Eighteen cadaveric specimens were randomized for bone density and diameter in the midclavicle section. Joint stiffness was measured by applying an axial load and a defined physiological range of motion for internal and external rotations and upward and downward rotations. Data were recorded at 3 stages: for the native joint after dissecting the AC ligaments, directly after repair, and after axial cyclic loading (1000 cycles with 20 and 70 N at 1 Hz). To evaluate which implant mimics physiological joint properties best, axial stiffness of vertical stability was assessed in combination with rotation. Finally, static loading in the superior direction was applied until failure of the joints occurred. Results: Axial stiffness of the TR and AS groups was 2-fold higher than for the HP group and the native joint (67.1, 66.1, and 22.5 N/mm, respectively; P < .004). Decreased load-to-failure rates were recorded in the HP group compared with the TR and AS groups (248.9 ± 72.7, 832.0 ± 401.4, and 538.0 ± 166.1 N, respectively). The stiffness of the rotations was not significantly different between the treatment methods but was lower in horizontal and downward rotations compared with the native state. Thus, native AC ligaments contributed a significant share to joint stiffness. Conclusion: The TR and AS groups demonstrated higher vertical load capacity. Compared with the TR and AS, the HP demonstrated an axial stiffness closest to the native joint. For restoring physiological properties, reconstruction of the AC ligaments may be necessary. Clinical Relevance: The results show different biomechanical properties of the HP and anatomic reconstructions.

Micromotions at the taper interface between stem and neck adapter of a bimodular hip prosthesis during activities of daily living.

The stem–neck taper interface of bimodular hip endoprostheses bears the risk of micromotions that can result in ongoing corrosion due to removal of the passive layer and ultimately cause implant fracture. We investigated the extent of micromotions at the stem–neck interface and the seating behavior of necks of one design made from different alloys during daily activities. Modular hip prostheses (n = 36, Metha®, Aesculap AG, Germany) with neck adapters (CoCr29Mo6 or Ti6Al4V) were embedded in PMMA (ISO 7206‐4) and exposed to cyclic loading with peak loads ranging from walking (Fmax = 2.3 kN) to stumbling (Fmax = 5.3 kN). Translational and rotational micromotions at the taper interface and seating characteristics during assembly and loading were determined using four eddy‐current sensors. Seating during loading after implant assembly was dependent on load magnitude but not on material coupling. Micromotions in the stem–neck interface correlated positively with load levels (CoCr: 2.6–6.3 µm, Ti: 4.6–13.8 µm; p < 0.001) with Ti neck adapters exhibiting significantly larger micromotions than CoCr (p < 0.001). These findings explain why high body weights and activities related to higher loads could increase the risk of fretting‐induced implant failures in clinical application, especially for Ti–Ti combinations. Still, the role of taper seating is not clearly understood.

Cement augmentation of the proximal femoral nail antirotation for the treatment of osteoporotic pertrochanteric fractures—A biomechanical cadaver study

INTRODUCTION Proximal femoral fractures will gain increasing importance in the future due to the epidemiological development. Osteoporosis is often a limiting factor in the achievement of implant stability. New nailing systems offer the possibility of augmentation of the femoral neck component with cement. The aim of this study was to perform a biomechanical comparison of implant stability in osteoporotic pertrochanteric fractures using the proximal femoral nail antirotation (PFNA, Synthes GmbH, Umkirch, Germany) with cement augmented and non-augmented blades. MATERIALS AND METHODS Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DEXA) in six pairs of fresh-frozen human femurs. Standardised pertrochanteric fractures (AO31-A2.3) were treated with a PFNA. Cement augmentation was performed in six constructs. Axial loading was applied according to a single-leg-stance model using a hydraulic testing machine increasing to 1400N over 10,000 cycles. Biomechanical comparisons between the two groups that were comparable concerning BMD, tip-apex-distance and native stiffness were made with regard to postoperative stiffness, survived cycles, load to failure, failure mechanism and axial displacement. RESULTS The stiffness of all stabilised femurs was significantly lower than for native specimens (native 702.5±159.6N/mm vs. postoperative 275.4±53.8N/mm, p<0.001). Stiffness after instrumentation was significantly greater for the cement augmented group than for the non-augmented group (300.6±46.7N/mm vs. 250.3±51.6N/mm, respectively, p=0.001). Five of the twelve constructs survived cyclic testing. Statistically significant differences of the BMD were detected between survived and failed constructs (0.79±0.17g/cm(2) vs. 0.45±0.12g/cm(2), respectively, p=0.028). The failure loads for specimens surviving 10,000 cycles were 4611.9±2078.9N in the cement augmented group (n=3) and 4516.3N and 3253.5N in the non-augmented group (n=2). Postoperative stiffness was found to be a positive predictor of maximum force to failure (R(2)=0.83, p=0.02). CONCLUSIONS The results of this biomechanical study show that cement augmentation of the PFNA increases the implant stability in osteoporotic pertrochanteric fractures. Further studies are necessary to evaluate this procedure in providing long term clinical results.

Einbringung von Pedikelschrauben unter Einsatz unterschiedlicher Unterstützungsverfahren

ZusammenfassungEin bekanntes Problem bei der thorakolumbalen dorsalen Spondylodese ist die Schraubenperforation durch die Pedikelkortikalis. Sie wird in bis zu 40% der eingebrachten Schrauben beschrieben. Zur Reduzierung dieser Perforationsrate wurden in den vergangenen Jahren CT- (Computertomographie-)basierende Navigationssysteme eingeführt, die dem Operateur die mehrdimensionale Kontrolle der Schraubenlage während der Operation in einer virtuellen Realität ermöglichen. In jüngster Vergangenheit wurden auch fluoroskopiebasierende Navigationssysteme entwickelt.Wir haben 77 Pedikelschrauben in humane lumbale WS- (Wirbelsäulen-)Präparate eingebracht, wobei zur Unterstützung entweder konventionelle Durchleuchtung, CT-basierte Navigation oder Fluoroskopie basierte Navigation eingesetzt wurde. Bei den kritischen Pedikelweiten zwischen 6,5 und 9 mm haben wir die besten Ergebnisse unter Verwendung der CT-basierenden Navigation gesehen, es fand sich zwischen den 3 Methoden jedoch kein signifikanter Unterschied.Wir sind der Überzeugung, dass der Schraubendurchmesser und der minimale Pedikeldurchmesser zukünftig in allen Arbeiten zur Perforationsrate von Pedikelschrauben und zur Effizienz von Navigationssystemen angegeben werden sollten, da diese nach unseren Ergebnissen essentielle Parameter bei der Perforationsrate von Pedikelschrauben darstellen.AbstractA well-known problem occurring with thoracolumbar spondylodesis is the perforation of pedicle screws through the pedicle wall. It occurs in up to 40% of the implanted screws. To reduce this problem, computed tomography (CT)-based navigation systems have been introduced, which allow the surgeon multidimensional control of the screw position in virtual reality and real time during insertion. In the recent past, fluoroscopy-based navigation systems have also been built.We inserted 77 pedicle screws in human lumbar cadaveric spine specimens either without navigation, with CT-based navigation, or with fluoroscopy-based navigation. In the critical sizes of pedicles between 6.5 and 9 mm, we found the best results with CT-based navigation, but there was no significant difference between the three methods.The minimal pedicle and the screw diameters should be reported in every study on pedicle screw misplacement and spine navigation since they represent the most important factor in pedicle wall perforations.

Design parameters and the material coupling are decisive for the micromotion magnitude at the stem–neck interface of bi-modular hip implants

Several bi-modular hip prostheses exhibit an elevated number of fretting-related postoperative complications most probably caused by excessive micromotions at taper connections. This study investigated micromotions at the stem-neck interface of two different designs: one design (Metha, Aesculap AG) has demonstrated a substantial number of in vivo neck fractures for Ti-Ti couplings, but there are no documented fractures for Ti-CoCr couplings. Conversely, for a comparable design (H-Max M, Limacorporate) with a Ti-Ti coupling only one clinical failure has been reported. Prostheses were mechanically tested and the micromotions were recorded using a contactless measurement system. For Ti-Ti couplings, the Metha prosthesis showed a trend towards higher micromotions compared to the H-Max M (6.5 ± 1.6 μm vs. 3.6 ± 1.5 μm, p=0.08). Independent of the design, prostheses with Ti neck adapter caused significantly higher interface micromotions than those with CoCr ones (5.1 ± 2.1 μm vs. 0.8 ± 1.6 μm, p=0.001). No differences in micromotions between the Metha prosthesis with CoCr neck and the H-Max M with Ti neck were observed (2.6 ± 2.0 μm, p=0.25). The material coupling and the design are both crucial for the micromotions magnitude. The extent of micromotions seems to correspond to the number of clinically observed fractures and confirm the relationship between those and the occurrence of fretting corrosion.

Malpositioning of the lag screws by 1- or 2-screw nailing systems for pertrochanteric femoral fractures: a biomechanical comparison of gamma 3 and intertan.

OBJECTIVES: The aim of this investigation was to perform a biomechanical comparison between one- and two-screw-systems used for the treatment of intertrochanteric fractures for centralized and decentralized placement of femoral-neck-screws in terms of failure loads, stiffness, survival rates, tip apex distance (TAD) and failure mode. METHODS: As fracture model, an AO 31A2.3 fracture was used. 12 pairs of human cadaver femora were tested. Femoral-neck-screws were implanted in the femoral head in center/center, posterior/central, and anterior/superior position in axial/frontal plane. A single-screw-system (Gamma 3 Locking Nail, Stryker) and a two-screw-system (Trigen-Intertan, Smith & Nephew) were used. To simulate the load in-situ, a cyclic load was carried for 10,000 cycles in a material-testing-machine. If no cyclic failure occurred, femora were loaded until failure. The systems were compared according stiffness, survivability through 10k cycles, TAD and load to failure. RESULTS: None of the tested bones failed at center/center location, in the decentralized positions 3 GammaNail and 2 Intertan specimens failed during cyclic testing. The two-screw-system resisted higher forces in all positions (Gamma: 5370N±1924, Intertan: 7650N±2043, p=0.014). CONCLUSIONS: Based on these data it is clear that both nail systems showed a higher biomechanical stability with a lower TAD. The two specimens that failed with the Intertan in the cyclic tests had a TAD ≥ 49mm. The cut-out failures that we detected during cyclic testing in the Gamma system had a TAD of ≥ 30mm. Thus it is clear that the TAD affects failure independent of the implant used. With a less than ideal lag screw placement however, the Intertan system with two integrated screws, was able to withstand higher loads in our study.

Zur Definition der Pedikelfehllage

ZusammenfassungVöllig fehlplatzierte Pedikelschrauben können zu Stabilitätsverlusten und zum Versagen der Montage führen. Es existieren keine internationalen Studien zum Stabilitätsverhalten von Pedikelschrauben, die nicht rein spongiös gelagert sind und den Pedikel nur gering (≤4 mm) überschreiten. Da neurologische Komplikationen nur bei Schrauben beschrieben werden, die die mediale Pedikelkortikalis um >4 mm überschreiten, hat diese Fragestellung eine große Bedeutung bei der Definition der Pedikelfehllage: 68 Pedikelschrauben wurden in humanen lumbalen Wirbelpräparaten 1000 sinusoidalen senkrechten Belastungszyklen mit einer Amplitude von 160 N ausgesetzt und der Schraubenweg am Anfang und Ende der Versuchsreihe gemessen. Die Schrauben wurden in 4 Gruppen entsprechend ihrer Lage im Pedikel unterteilt. Wir haben zwischen den 4 Gruppen keine signifikanten Unterschiede im Lockerungsverhalten festgestellt. Die beiden Gruppen Kortikaliskontakt und Fehllage wiesen im Vergleich zu den beiden anderen Gruppen ein tendenziell besseres Ergebnis auf, welches jedoch nicht signifikant war.AbstractTotally misplaced pedicle screws will lead to major stability problems. There are no publications about the stability behavior of screws, which have no pure trabecular position and perforate the pedicle slightly. Since neurological problems are only described with screws perforating the pedicle medially by at least 4 mm, this question has great relevance concerning the definition of pedicular malposition. Sixty-eight pedicle screws were tested in human cadaveric lumbar spines. Their vertical path was measured at the beginning and end of 1000 sinusoidal cycles with a force amplitude of 160 N. They were divided into four groups according to their screw position as mentioned in the title. We found no significant differences in primary stability or loosening between the groups. The cortical contact and perforation groups had slightly better results, which were not significant in comparison to the other two groups.

Assembly force and taper angle difference influence the relative motion at the stem-neck interface of bi-modular hip prostheses.

Bi-modular hip arthroplasty prostheses allow adaptation to the individual patient anatomy and the combination of different materials but introduce an additional interface, which was related lately to current clinical issues. Relative motion at the additional taper interface might increase the overall risk of fretting, corrosion, metallic debris and early failure. The aim of this study was to investigate whether the assembly force influences the relative motion and seating behaviour at the stem–neck interface of a bi-modular hip prosthesis (Metha®; Aesculap AG, Tuttlingen, Germany) and whether this relation is influenced by the taper angle difference between male and female taper angles. Neck adapters made of titanium (Ti6Al4V) and CoCr (CoCr29Mo) were assembled with a titanium stem using varying assembly forces and mechanically loaded. A contactless eddy current measurement system was used to record the relative motion between prosthesis stem and neck adapter. Higher relative motion was observed for Ti neck adapters compared to the CoCr ones (p < 0.001). Higher assembly forces caused increased seating distances (p < 0.001) and led to significantly reduced relative motion (p = 0.019). Independent of neck material type, prostheses with larger taper angle difference between male and female taper angles exhibited decreased relative motion (p < 0.001). Surgeons should carefully use assembly forces above 4 kN to decrease the amount of relative motion within the taper interface. Maximum assembly forces, however, should be limited to prevent periprosthetic fractures. Manufacturers should optimize taper angle differences to increase the resistance against relative motion.

Locking plate fixation of humeral head fractures with a telescoping screw. A comparative biomechanical study versus a standard plate

OBJECTIVES Locking plate fixation of humeral head fractures bares the risk of glenohumeral screw penetration. In order to circumvent this problem it is recommended to insert shorter locking screws having at least a 6mm distance to the humeral head cortex. This in turn may reduce fixation stability and may lead to early varus displacement. One second frequent failure mechanism is cranial displacement of the greater tubercle. The study evaluates the biomechanical properties of a locking plate employing an additional telescoping screw that may enhance resistance to varus displacement. Screw in screw fixation of the greater tubercle may reduce the rate of cranial displacement. METHODS In four paired fresh-frozen human cadaver humeri (age>70 years) a Neer IV/3 fracture was created with a 5mm osteotomy gap simulating metaphyseal comminution. Limbs were randomly assigned to receive plate fixation with an additional telescoping screw (Humerus Tele Screw: HTS) and on the contralateral limb Philos plate fixation before biomechanical evaluation (MTS-Bionix 858.2). Standard locking screws were placed in both groups 6mm below the radiological head circumference; the telescoping screw was placed in the subchondral layer. The greater tubercle was fixed with an additional screw in both techniques, in the HTS group the screw was anchored in the sleeve of the telescrew (screw in screw fixation). FINDINGS Fixation stability with a mean stiffness of 300.9±28.8 N/mm in the HTS plate group proved to be significantly higher than in the Philos plate group (184.2±23.4 N/mm; p=0.006). The HTS plate also resisted higher loads in terms of fixation failure with loss of reduction at 290±58.6 N in comparison to 205±8.6 N for the Philos plate (p=0.2). Displacement of the greater tubercle occurred in no case of the HTS plate group and in two out of four cases in the Philos plate group. INTERPRETATION The HTS plate provides high fixation stability in an in vitro humeral head fracture model and securely prevents displacement of the greater tubercle.

The role of inter-prosthetic distance, cortical thickness and bone mineral density in the development of inter-prosthetic fractures of the femur

It is becoming increasingly common for a patient to have ipsilateral hip and knee replacements. The inter-prosthetic (IP) distance, the distance between the tips of hip and knee prostheses, has been thought to be associated with an increased risk of IP fracture. Small gap distances are generally assumed to act as stress risers, although there is no real biomechanical evidence to support this. The purpose of this study was to evaluate the influence of IP distance, cortical thickness and bone mineral density on the likelihood of an IP femoral fracture. A total of 18 human femur specimens were randomised into three groups by bone density and cortical thickness. For each group, a defined IP distance of 35 mm, 80 mm or 160 mm was created by choosing the appropriate lengths of component. The maximum fracture strength was determined using a four-point bending test. The fracture force of all three groups was similar (p = 0.498). There was a highly significant correlation between the cortical area and the fracture strength (r = 0.804, p < 0.001), whereas bone density showed no influence. This study suggests that the IP distance has little influence on fracture strength in IP femoral fractures: the thickness of the cortex seems to be the decisive factor.