A Biomechanical Comparison of Allograft Tendons for Ligament Reconstruction.

BACKGROUND: Allograft tendons are frequently used for ligament reconstruction about the knee, but they entail availability and cost challenges. The identification of other tissues that demonstrate equivalent performance to preferred tendons would improve limitations. Hypothesis/Purpose: We compared the biomechanical properties of 4 soft tissue allograft tendons: tibialis anterior (TA), tibialis posterior (TP), peroneus longus (PL), and semitendinosus (ST). We hypothesized that allograft properties would be similar when standardized by the looped diameter. STUDY DESIGN: Controlled laboratory study. METHODS: This study consisted of 2 arms evaluating large and small looped-diameter grafts: experiment A consisted of TA, TP, and PL tendons (n = 47 each) with larger looped diameters of 9.0 to 9.5 mm, and experiment B consisted of TA, TP, PL, and ST tendons (n = 53 each) with smaller looped diameters of 7.0 to 7.5 mm. Each specimen underwent mechanical testing to measure the modulus of elasticity (E), ultimate tensile force (UTF), maximal elongation at failure, ultimate tensile stress (UTS), and ultimate tensile strain (UTε). RESULTS: Experiment A: No significant differences were noted among tendons for UTF, maximal elongation at failure, and UTϵ. UTS was significantly higher for the PL (54 MPa) compared with the TA (44 MPa) and TP (43 MPa) tendons. E was significantly higher for the PL (501 MPa) compared with the TP (416 MPa) tendons. Equivalence testing showed that the TP and PL tendon properties were equivalent or superior to those of the TA tendons for all outcomes. Experiment B: All groups exhibited a similar E. UTF was again highest in the PL tendons (2294 N) but was significantly different from only the ST tendons (1915 N). UTϵ was significantly higher for the ST (0.22) compared with the TA (0.19) and TP (0.19) tendons. Equivalence testing showed that the TA, TP, and PL tendon properties were equivalent or superior to those of the ST tendons. CONCLUSION: Compared with TA tendons, TP and PL tendons of a given looped diameter exhibited noninferior initial biomechanical strength and stiffness characteristics. ST tendons were mostly similar to TA tendons but exhibited a significantly higher elongation/UTϵ and smaller cross-sectional area. For smaller looped-diameter grafts, all tissues were noninferior to ST tendons. In contrast to previous findings, PL tendons proved to be equally strong. CLINICAL RELEVANCE: The results of this study should encourage surgeons to use these soft tissue allografts interchangeably, which is important as the number of ligament reconstructions performed with allografts continues to rise.

Biomechanical and Cost Comparisons of Near-Far and Pin-Bar Constructs.

Orthopedic dogma states that external fixator stiffness is improved by placing 1 pin close to the fracture and 1 as distant as possible ("near-far"). This fixator construct is thought to be less expensive than placing pins a shorter distance apart and using "pin-bar" clamps that attach pins to outriggers. The authors therefore hypothesized that the near-far construct is stiffer and less expensive. They compared mechanical stiffness and costs of near-far and pin-bar constructs commonly used for temporary external fixation of femoral shaft fractures. Their testing model simulated femoral shaft fractures in damage control situations. Fourth-generation synthetic femora (n=18) were used. The near-far construct had 2 pins that were 106 mm apart, placed 25 mm from the gap on each side of the fracture. The pin-bar construct pins were 55 mm apart, placed 40 mm from the gap. Mechanical testing was performed on a material test system machine. Stiffness was determined in the linear portion of the load-displacement curve for both constructs in 4 modes: axial compression, torsional loading, frontal plane 3-point bending, and sagittal plane 3-point bending. Costs were determined from a 2012 price guide. Compared with the near-far construct, the pin-bar construct had stiffness increased by 58% in axial compression (P<.05) and by 52% in torsional loading (P<.05). The pin-bar construct increased cost by 11%. In contrast to the authors' hypothesis and existing orthopedic dogma, the near-far construct was less stiff than the pin-bar construct and was similarly priced. Use of the pin-bar construct is mechanically and economically reasonable. [Orthopedics. 2017; 40(2):e238-e241.].

A Biomechanical Analysis of Anchor Placement for Bankart Repair: Effect of Portal Placement.

During arthroscopic Bankart repair, penetration of suture anchors through the far cortex can compromise the initial biomechanical characteristics of anchor stability and repair integrity. This study compared the placement of suture anchors through a low anterior-inferior rotator interval portal (AI) vs a trans-subscapularis portal to evaluate the rate of anchor perforation as well as biomechanical strength. Ten matched pairs of cadaveric shoulders were randomized to an AI or a trans-subscapularis portal for placement of suture anchors at the 3 o'clock and 5:30 positions. The following measurements were obtained: (1) distance from the portal to the cephalic vein; (2) presence and length of anchor penetration through the inferior glenoid; and (3) ultimate failure strength of the anchors. The distance from the portal to the cephalic vein was significantly greater with the AI vs the trans-subscapularis portal across all specimens (29.9 vs 11.2 mm, P<.05). The rate of anchor penetration was significantly increased in the AI group vs the trans-subscapularis group at the 5:30 position (60% vs 10%, P=.014) but not at the 3 o'clock position (P=.33). Mean pullout strength of the anchors at the 5:30 position trended higher in the trans-subscapularis group, but the difference was not significant (132.8 vs 112.6 N, P=.18). The cephalic vein is closer to the trans-subscapularis portal than to the AI, but is at a safe distance. Both the rate and the degree of glenoid suture anchor penetration were lower with the trans-subscapularis portal compared with the AI at the 5:30 position. Placing anchors through the trans-subscapularis portal provides a safe alternative method, with improved positioning of the inferiormost anchor compared with the traditional AI.

Internal anterior fixators for pelvic ring injuries: Do monaxial pedicle screws provide more stiffness than polyaxial pedicle screws?

OBJECTIVES: Little is known about the mechanical properties of internal anterior fixators (known as INFIX), which have been proposed as subcutaneous alternatives to traditional anterior external fixators for pelvic ring disruptions. We hypothesised that INFIX has superior biomechanical performance compared with traditional external fixators because the distance from the bar to the bone is reduced. METHODS: Using a commercially available synthetic bone model, 15 unstable pelvic ring injuries were simulated by excising the pubic bone through the bilateral superior and inferior rami anteriorly and the sacrum through the bilateral sacral foramen posteriorly. Three test groups were established: (1) traditional supra-acetabular external fixation, (2) INFIX with polyaxial screws, (3) INFIX with monaxial screws. Load was applied, simulating lateral compression force. Outcome measure was construct stiffness. RESULTS: The traditional external fixator constructs had an average stiffness of 6.21 N/mm ± 0.40 standard deviation (SD). INFIX with monaxial screws was 23% stiffer than the traditional external fixator (mean stiffness, 7.66 N/mm ± 0.86 SD; p = .01). INFIX with polyaxial screws was 26% less stiff than INFIX with monaxial screws (mean stiffness, 5.69 N/mm ± 1.24 SD; p = .05). No significant difference was noted between polyaxial INFIX and external fixators (mean stiffness, 6.21 N/mm ± 0.40 SD; p=.65). CONCLUSIONS: The performance of INFIX depends on the type of screw used, with monaxial screws providing significantly more stiffness than polyaxial screws. Despite the mechanical advantage of being closer to the bone, polyaxial INFIX was not stiffer than traditional external fixation.

Rib Fracture Fixation Restores Inspiratory Volume and Peak Flow in a Full Thorax Human Cadaveric Breathing Model.

BACKGROUND: Multiple rib fractures cause significant pain and potential for chest wall instability. Despite an emerging trend of surgical management of flail chest injuries, there are no studies examining the effect of rib fracture fixation on respiratory function. OBJECTIVES: Using a novel full thorax human cadaveric breathing model, we sought to explore the effect of flail chest injury and subsequent rib fracture fixation on respiratory outcomes. PATIENTS AND METHODS: We used five fresh human cadavers to generate negative breathing models in the left thorax to mimic physiologic respiration. Inspiratory volumes and peak flows were measured using a flow meter for all three chest wall states: intact chest, left-sided flail chest (segmental fractures of ribs 3 - 7), and post-fracture open reduction and internal fixation (ORIF) of the chest wall with a pre-contoured rib specific plate fixation system. RESULTS: A wide variation in the mean inspiratory volumes and peak flows were measured between specimens; however, the effect of a flail chest wall and the subsequent internal fixation of the unstable rib fractures was consistent across all samples. Compared to the intact chest wall, the inspiratory volume decreased by 40 ± 19% in the flail chest model (P = 0.04). Open reduction and internal fixation of the flail chest returned the inspiratory volume to 130 ± 71% of the intact chest volumes (P = 0.68). A similar 35 ± 19% decrease in peak flows was seen in the flail chest (P = 0.007) and this returned to 125 ± 71% of the intact chest following ORIF (P = 0.62). CONCLUSIONS: Negative pressure inspiration is significantly impaired by an unstable chest wall. Restoring mechanical stability of the fractured ribs improves respiratory outcomes similar to baseline values.

Bar diameter is an important component of knee-spanning external fixator stiffness and cost.

The authors' objective was to determine the effects of bar diameter on the stiffness and cost of a knee-spanning external fixator. The authors studied 2 versions of an external fixator with a difference in bar diameter (small bars, 8-mm diameter; large bars, 11-mm diameter). Fixators were tested using frame dimensions and a synthetic fracture model appropriate for a tibial plateau fracture. Five configurations of each fixator were tested: standard, cross-link, oblique pin, double stack, and super construct. The construct stiffness of each configuration (n=60) was measured in anterior-posterior bending, medial-lateral bending, axial torsion, and axial compression. Cost analysis allowed for calculation of the stiffness per unit cost. In the large bar group, an increase in construct stiffness was noted for all constructs and testing modes. Magnitude of stiffness increase ranged from 24% to 224% (P<.05 in all cases), depending on the configuration and loading mode. Increase in stiffness was so large that double-stack small bars performed similarly to standard construct large bars. Considering that the frame components have similar costs, the double-stack small bar fixator results in a 66% increase in cost for the same stiffness provided by the standard large bar. Bar diameter seems to have a large effect on knee-spanning external fixators. The authors observed an increase in stiffness of up to 191% under anterior-posterior bending despite an increase in bar size of only 37.5%. This finding might allow clinicians to use less expensive frames constructed of larger bars without sacrificing construct stiffness.

Analysis of strategies to increase external fixator stiffness: is double stacking worth the cost?

We compared the mechanical benefits and costs of 3 strategies that are commonly used to increase knee-spanning external fixator stiffness (resistance to deformation): double stacking, cross-linking, and use of an oblique pin. At our academic trauma centre and biomechanical testing laboratory, we used ultra-high-molecular-weight polyethylene bone models and commercially available external fixator components to simulate knee-spanning external fixation. The models were tested in anterior-posterior bending, medial-lateral bending, axial compression, and torsion. We recorded the construct stiffness for each strategy in all loading modes and assessed a secondary outcome of cost per 10% increase in stiffness. Double stacking significantly increased construct stiffness under anterior-posterior bending (109%), medial-lateral bending (22%), axial compression (150%), and torsion (41%) (p<0.05). Use of an oblique pin significantly increased stiffness under torsion (25%) (p<0.006). Cross-linking significantly increased stiffness only under torsion (29%) (p<0.002). Double stacking increased costs by 84%, cross-linking by 28%, and use of an oblique pin by 15% relative to a standard fixator. All 3 strategies increased stiffness under torsion to varying degrees, but only double stacking increased stiffness in all 4 testing modalities (p<0.05). Double stacking is most effective in increasing resistance to bending, particularly under anterior-posterior bending and axial compression, but requires a relatively high cost increase. Clinicians can use these data to help guide the most cost-effective strategy to increase construct stiffness based on the plane in which stiffness is needed.