Biomechanical analysis of dual versus lateral locked plating in elderly bicondylar tibial plateau fractures: Does medial comminution matter?

OBJECTIVE: To assess dual plating versus lateral locked plate fixation of bicondylar tibial plateau fractures in an elderly cadaveric model with and without medial bone loss PARTICIPANTS: 10 pairs of elderly (range 78-93 years of age) fresh frozen tibias. INTERVENTION: Bicondylar tibial plateau fractures were created reproducing AO/OTA 41 C1 (without medial bone loss) and C2 fractures (with medial bone loss). Cadavers were randomized to 4 different groups. Groups 1 and 2 were 41 C1 fractures and fixated with either dual or lateral plating, respectively. Groups 3 and 4 were fixated in a similar fashion with medial metaphyseal bone loss (41 C2 fracture) with dual plating Group 3 and lateral plating group 4. Lateral plating consisted of a 3.5 mm 5-hole lateral plate (ALPS, ZimmerBiomet) with 6 bicortical locking screws proximally and two diaphyseal screws. Dual plating groups underwent lateral plating and additional 3.5 mm 5-hole posteromedial 1/3 tubular plate (ZimmerBiomet) placed at the apex of the fracture with two shaft screws and one unicortical screw proximally. Specimens were tested in a Mechanical Testing System (MTS) machine loading both condyles. OUTCOME MEASUREMENTS: Specimens were loaded to 300 N and coronal alignment obtained. Specimens were then cycled from 100 N to 700 N for 5000 cycles at 2 Hz. Average axial displacement, maximal displacement, average force and coronal alignment after 5000 cycles were recorded. Lastly, force to failure was recorded at 100 N/sec. RESULTS: Mean axial displacement was 4.21 mm ranging from 3.12 mm in group 1 to 5.92 mm in group 4 (P = 0.51). Failure force averaged 3340 N ranging from 4342 N in group 1 to 2433 N in group 4 (P = 0.36). Maximal displacement ranged from 3.69 mm in group 1 to 7.37 mm in group 4 (P = 0.21). Change in coronal alignment ranged from 0.98° in group 1 to 1.97° in group 4 (P = 0.45). No statistically significant difference was noted between all four groups for all data points. CONCLUSION: The results of this study demonstrate that a lateral locked plate may offer an alternative means of fixation in AO/OTA 41 C1 and C2 fractures.

Hybrid screw fixation for femoral neck fractures: Does it prevent mechanical failure?

INTRODUCTION: Traditionally, femoral neck fracture fixation has been performed using three partially threaded cancellous screws. However, fracture collapse with femoral neck shortening, and varus deformation frequently occurs due to posterior medial comminution and lack of calcar support. We hypothesize replacing the inferior neck/calcar screw with a fully threaded, length stable, screw will provide improved biomechanical stability, decrease femoral neck shortening and varus collapse. METHODS: Ten matched cadaveric pairs (20 femurs) were randomly assigned to two screw fixation groups. Group 1 (Hybrid) utilized one fully threaded calcar screw & two partially threaded superior screws. Group 2 (PT) utilized all partially threaded screws. Specimens underwent standardized femoral neck osteotomies, 45° from the horizontal, with 5 mm posteromedial wedge removed to simulate posteromedial comminution. Screws were placed using fluoroscopic guidance. Specimens were biomechanically tested using two loading sequences: 1) Axial load applied up to 700 N, followed by cyclic loading at 2 Hz with loads of 700 to 1,400 N for 10,000 cycles. 2) All surviving constructs were cyclically loaded to failure in stepwise incremental manner with max load of 4,000 N. Paired t-tests used to compare stiffness, cycles to failure, and max load to failure (defined as 15 mm load actuator displacement). RESULTS: Construct stiffness was 2848 ± 344 N/mm in PT vs. 2767 ± 665 for Hybrid (P = 0.628). Load to failure demonstrated, hybrid superiority with max cycles to failure (3797 ± 400 cycles) vs. (2981 ± 856 cycles in PT) (p = 0.010), and max load prior to failure (3290 ± 196 N) vs. (2891 ± 421 N in PT) (p = 0.010). No significant difference in bone mineral density was noted in any of the specimens. CONCLUSIONS: Our study is the first to assess the biomechanical effects of hybrid fixation for femoral neck fractures. Hybrid screw configuration resulted in significantly stronger constructs, with higher axial load and increased cycles prior to failure. The advantageous mechanical properties demonstrated using a fully threaded inferior calcar screw provides a length stable construct which may prevent the common complication of excessive femoral neck shortening, varus collapse and poor functional outcome.

Biomechanical evaluation of medial malleolus fractures treated with headless compression screws.

PURPOSE: The gold standard for fixation of medial malleolus fractures has yet to be determined. Most agree the best results of displaced unstable ankle fractures are with open reduction and internal fixation. Hardware irritation necessitating screw removal is a known complication. An alternative fixation method of medial malleolar fractures has been described using headless compression screws. There are currently no biomechanical studies that assess the stability of this method. The purpose of the study is to complete a biomechanical evaluation of partially threaded cancellous screws (PT) and headless compression screws (HC) in an external rotation ankle fracture model. METHODS: Composite polyurethane sawbone models of tibia were obtained. A custom jig was created to ensure identical osteotomies. The models were fixed with either two partially threaded cancellous screws or two headless compression screws. The models were fitted into the Material Test System (MTS) machine and the force transducer was programed to apply axial offset. The two constructs were loaded until 5 mm of displacement occurred, our defined point of failure. The amount of force (Newtons) necessary to create fracture line displacement was recorded for each model. RESULTS: The axial rotational force to create facture displacement between 1 mm and 3 mm, between 1 mm and 5 mm, and between 3 mm and 5 mm were statistically significant (p < 0.05) for both models. The mean axial rotational force measured in Newtons at 1 mm, 3 mm, and 5 mm of displacement for the PT was 149.32, 244.19 and 477.76 respectively. The mean axial rotational force measured in Newtons at 1 mm, 3 mm, and 5 mm of displacement for the HC was 152.05, 224.07, and 498.31 respectively. CONCLUSION: No statistically significant difference was found between the biomechanical properties of partially threaded cancellous screws and headless compression screws used in the fixation of medial malleolus fractures. These results support HC screws as a viable alternative in a biomechanical model of medial malleolus fractures.

Nonanatomic Tibial Tunnel Placement for Single-Bundle Posterior Cruciate Ligament Reconstruction Leads to Greater Posterior Tibial Translation in a Biomechanical Model.

PURPOSE: To determine the effect of varying proximal-distal tibial tunnel placement on posterior cruciate ligament (PCL) laxity. METHODS: Nine matched pairs (18 total) of cadaveric knees (mean age 79.3 years, range 60 to 89), were studied. The specimens from each pair were randomly divided into 2 groups based on tibial tunnel placement: (1) anatomic tunnel and (2) proximal nonanatomic tunnel. A 150-N cyclic posterior tibial load was applied using a Materials Testing System machine at 0°, 30°, 60°, and 90° of knee flexion. Each specimen completed 50 cycles at a rate of 0.2 Hz at each knee flexion angle. In 10 specimens, a static 250-N posterior tibial load was applied at 90° of knee flexion. Posterior tibial translation was recorded. Load to failure for all specimens was recorded. RESULTS: With application of a 150-N posteriorly directed cyclic force, the anatomic tunnel group had significantly less posterior tibial translation (millimeters, mean [standard deviation (SD)]) than the proximal nonanatomic tunnel group at 0°, 30°, 60°, and 90° of knee flexion: 1.1 (0.3) v 1.5 (0.4), P = .031; 1.1 (0.6) v 2.2 (0.9), P = .019; 0.9 (0.4) v 2.0 (0.6), P = .001; 0.9 (0.6) v 2.9 (0.7), P < .001, respectively. The anatomic tunnel group also demonstrated significantly less posterior tibial translation (millimeters, mean [SD]) than the nonanatomic tunnel group at 90° with a static 250-N posteriorly directed force applied (P <.05): 2.3 (1.3) v 6.1 (2.3), P = .016. Four pairs were excluded from the 250-N results because of prior load to failure testing. CONCLUSIONS: Anatomic tibial tunnel placement re-creating the tibial origin of the PCL results in significantly less posterior tibial translation than proximal nonanatomic tibial tunnel placement. Correct placement of the tibial tunnel during PCL reconstruction is essential for avoidance of posterior laxity. CLINICAL RELEVANCE: Anatomic tibial tunnel placement during PCL reconstruction may ensure a more stable reconstruction.

Biomechanical Contribution of Tension-Reducing Rotator Cuff Sutures in 3-Part Proximal Humerus Fractures.

OBJECTIVES: Using a cadaveric 3-part fracture model and cyclic loading protocol, our study objectives were to quantify the stabilizing effect of tension-reducing rotator cuff sutures in terms of fracture displacement across the surgical neck and greater tuberosity compared with a control group in which no sutures were used. METHODS: Six matched pairs of fresh frozen specimens underwent a standardized, 3-part, proximal humerus fracture and were split into 2 groups. The control group had the fracture fixed with a plate and screw construct only while the experimental group had additional suture fixation through the plate to each rotator cuff tendon. Active abduction through the rotator cuff was simulated for 100, 200, 300, and 400 cycles and to failure at 1000 N. A Mann-Whitney U test compared cyclic displacement of the greater tuberosity and surgical neck fracture gaps and load to failure between the 2 groups. RESULTS: There was no significant difference (P > 0.05) in fracture gap between fixation methods at the surgical neck at 100 (P = 0.13), 200 (P = 0.07), 300 (P = 0.49), and 400 (P = 0.07) cycles. There was no significant difference (P > 0.05) between fixation methods in the fracture gap at the greater tuberosity at 100 (P = 0.39), 200 (P = 1.00), 300 (P = 0.31), and 400 (P = 0.59) cycles. There was no significant difference (P > 0.5) at 1000 N at the surgical neck (P = 0.70) or the greater tuberosity (P = 0.39). CONCLUSIONS: Tension-relieving rotator cuff sutures do not add stability to the repair of 3-part proximal humerus fractures. Varus collapse and greater tuberosity displacement are common complications associated with 3-part fractures. No mechanical data exist to demonstrate benefit of adding suture to a plate and screw construct for limiting fracture displacement.

Anterior Cruciate Ligament Graft Isometry is Affected by the Orientation of the Femoral Tunnel.

PURPOSE: The purpose of this study was to compare anterior cruciate ligament (ACL) graft length and tension throughout knee range of motion with transtibial, anteromedial (AM) portal, and all-epiphyseal drilling techniques with suspensory and apical femoral fixation. METHODS: The three different femoral tunnel drilling techniques using the same intra-articular starting point within the center of the femoral footprint were performed on fresh-frozen cadaveric specimens. All groups underwent standard tibial drilling in the center of the ACL tibial footprint. FiberWire (Arthrex Inc., Naples, FL) was used to simulate anatomic single bundle reconstructions. Changes in graft length and tension were measured at knee flexion angles of 0, 30, 60, 90, 120, and 135 degrees. RESULTS: Graft length and tension decreased from 0 through 60 degrees and subsequently increased from 90 to 135 degrees for all groups. The transtibial, AM portal suspensory, and apical fixation groups were similar. However, the all-epiphyseal tunnel with suspensory fixation had a significantly increased change in length (90, 120, and 135 degrees) and tension (120 and 135 degrees). CONCLUSION: Transtibial and AM portal suspensory fixation and apical fixation demonstrate similar changes in length and tension throughout knee range of motion. The all-epiphyseal tunnel with suspensory fixation was associated with greater length and tension changes at higher degrees of knee flexion. All techniques demonstrated decreased graft length and tension with knee flexion to 60 degrees after which they increased with further knee flexion. CLINICAL RELEVANCE: ACL graft length and tension change throughout knee range of motion and also depend on femoral tunnel orientation and fixation type. The use of an all-epiphyseal tunnel with suspensory fixation should be studied further for evidence of graft elongation.

No Contribution of Tension-Reducing Rotator Cuff Sutures on Locking Plate Fixation in a 2-Part Proximal Humerus Fracture Model.

OBJECTIVES: Varus failure is a well-known complication of open reduction internal fixation of proximal humeral fractures. The addition of tension-reducing sutures from the plate to the rotator cuff may attenuate the deforming forces of the rotator cuff resulting in decreased varus failure. In this study, we investigate the biomechanical contributions of tension-reducing sutures to a locked plate construct in a 2-part proximal humerus fracture model. METHODS: Two fixation techniques were tested in 12 matched fresh frozen humeri in which standard 2-part fractures of the surgical neck were created with a gap simulating surgical neck medial comminution. In group 1, fractures were fixed with a standard proximal humerus locking plate. In group 2, the plate fixation was similar, and additionally, tension-reducing sutures were applied from the plate to the rotator cuff. Active abduction was simulated for 400 cycles with force applied through the rotator cuff tendons. Intercyclic fracture motion, change in displacement, and load to failure were recorded. RESULTS: The addition of tension-reducing sutures did not lead to significant differences in intercyclic fracture motion. The mean change in displacement and load to failure were similar in both groups. Failure typically occurred in both groups at the rotator cuff testing clamp interface. CONCLUSIONS: Tension-relieving rotator cuff sutures added to locking plate fixation did not lead to a change in fracture gap with cyclic loading or an increase in ultimate failure load in a 2-part surgical neck proximal humerus fracture model without medial support.

Biomechanical comparison of long head of biceps tenodesis with interference screw and biceps sling soft tissue techniques.

PURPOSE: The purpose of our study was to compare biomechanically a long head biceps tenodesis using an all soft tissue biceps sling technique versus an interference screw technique. METHODS: Six paired fresh frozen shoulder specimens were separated into 2 groups. One group used an all soft tissue biceps sling technique for tenodesis. The other group used the interference screw technique for subpectoral tenodesis of the long head biceps tendon. Specimens in both groups were sequentially loaded for 200 cycles, and the difference between the initial and final displacements were recorded. Specimens were then loaded to failure. Load and mode of failure were recorded. RESULTS: The mean displacement of all specimens undergoing the sling technique was significantly less than that of the interference technique at 3.0 mm (±0.80) versus 5.0 mm (±1.08) (P < .05). The biceps sling technique had a higher mean ultimate failure load (UFL) than did the interference screw tenodesis (216.9 N ± 91.6 v 171.7 N ± 101.4), although this was not statistically significant (P = .63). In the interference screw technique, 4 specimens failed at the tenodesis site by either tearing or complete pullout, whereas 2 failed at the biceps myotendinous junction. In the sling technique, 4 specimens failed at the biceps myotendinous junction, whereas one specimen tore at the tenodesis site and one detached the pectoralis tendon insertion from the humerus. One specimen in the biceps sling technique and 2 specimens in the interference screw technique failed before completing all 200 cycles. CONCLUSIONS: The results of this biomechanical study show that the biceps sling technique has construct stability similar to that of the interference screw technique. CLINICAL RELEVANCE: The biceps sling may be a reasonable alternative for treating symptomatic pathologic conditions of the long head biceps tendon.

Relationship of cortical thickness of the proximal humerus and pullout strength of a locked plate and screw construct.

BACKGROUND: No study to date has evaluated cortical thickness as it relates to locking plate failure or screw pullout in the proximal humerus. The purpose of this study is to determine the relationship between proximal humerus cortical thickness and locked plate hardware failure in a cadaveric proximal humerus fracture model. METHODS: Twelve humerus specimens were placed into two groups based on the proximal humerus cortical thickness on an anteroposterior radiograph: less than 4 mm and greater than 4 mm. The specimens were plated with a six-hole proximal humerus locking plate and a 15-mm resection osteotomy at the surgical neck was performed. The specimens were tested in a materials testing machine at a displacement of 5 mm/min to failure. RESULTS: Load at failure, stiffness, maximum load, failure, and fracture gap closure were all statistically similar (P > 0.05) between the groups. CONCLUSION: Our biomechanical study used modern locked plate-screw construct fixation of a simulated two-part proximal humerus fracture. The mechanical strength was unaffected based on a threshold combined proximal humerus cortical thickness of 4 mm.

Circular external fixation frames with divergent half pins: a pilot biomechanical study.

The use of hexapod circular external fixators has simplified the ability to correct complex limb deformities without cumbersome frame reconfigurations. These frames are applied primarily using half pin mountings and may be difficult to utilize given the constraints of traditional half pin constructs. We compared the biomechanical performance of simplified divergent half pin frames to mountings currently being utilized for application of hexapod frames. Three 6-mm half pins per limb segment were placed into sawbones at 60 degree divergent angles in both the sagittal and coronal planes in a 2-cm diaphyseal fracture gap model. Pin mountings were attached to a standardized four-ring construct. This was compared to similar four-ring frames with two differing pin/wire configurations: (1) two tensioned wires per ring placed at 90 degree angles, a total eight wires; and (2) two 5-mm half pins per ring placed at 90 degree angles, a total eight half pins. The divergent 6-mm half pin frames demonstrated similar performance compared the standardized tensioned wire and 5-mm half pin frames in terms of axial micromotion and angular deflection. Based on the mechanical performance of these divergent half pin frames we believe they can be used clinically without detrimental consequences.