Biomedical subjects

Andrew Mahar

Publications and source records attributed to Andrew Mahar.

At least 19 recordsLinked to original sources

PubMed2007

Biomechanical analysis of femoral tunnel pull-out angles for anterior cruciate ligament reconstruction with bioabsorbable and metal interference screws.

BACKGROUND: Fixation strength of metal and bioabsorbable interference screws has not been evaluated while varying the anterior cruciate ligament graft tension angle. HYPOTHESIS: There is no difference in fixation strength between 2 types of interference screws for anterior cruciate ligament graft fixation while the graft tension angle is varied relative to the femoral tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: Forty-eight anterior cruciate ligament reconstructions were performed using immature porcine femurs stripped of soft tissue and doubled-over porcine flexor digitorum profundus tendon grafts. Specimens were randomized to bioabsorbable or titanium interference screw fixation. Specimens were randomized to one of three pull angles (0 degrees , 30 degrees , 60 degrees ) representing loading at different knee flexion angles (n = 8/group). Reconstructed ligaments were tensioned to 10 N followed by 200 loading cycles between 10 and 150 N and a final failure test. Construct elongation (mm) at 100 and 200 cycles and failure load (N) were analyzed using a 2-way analysis of variance (P < .05). RESULTS: Screw material interacted significantly with graft tension angle, as the bioabsorbable screw specimens demonstrated significantly greater fixation strength when tensioned at greater angles. Specimens fixed with bioabsorbable screws showed significantly less elongation at both 100 and 200 cycles and significantly greater failure load compared with titanium screws. CONCLUSION: Bioabsorbable interference screws acutely have increased fixation strength compared with titanium interference screws for anterior cruciate ligament grafts loaded at greater tension angles. CLINICAL RELEVANCE: The strength of anterior cruciate ligament reconstruction fixation increases with increasing divergence between the tension angle and femoral tunnel, a condition seen when the knee approaches full extension.

Absorbable Implants↗

PubMed2007

Titanium versus stainless steel for anterior spinal fusions: an analysis of rod stress as a predictor of rod breakage during physiologic loading in a bovine model.

STUDY DESIGN: In vitro biomechanical evaluation of rod stress during physiologic loading of anterior scoliosis instrumentation. OBJECTIVES: To determine effects of material properties and rod diameter on rod stresses in anterior scoliosis instrumentation. SUMMARY OF BACKGROUND DATA: Relationships between instrumentation dimensions, materials, and potential rod failure in anterior scoliosis instrumentation remain unclear. METHODS: Eighteen immature bovine spines were randomized to 3 groups: 1) 4.0-mm stainless steel, 2) 5.0-mm stainless steel, and 3) 4.75-mm titanium alloy. Spines underwent physiologic tests in flexion-extension, lateral bending, and torsion. Rod surface strains were converted to rod stress and normalized to each material's yield stress. Construct stiffness and the normalized rod stresses were compared with a one-way ANOVA (P < 0.05). RESULTS: The 4.0-mm steel and 4.75-mm titanium construct stiffness was similar across all tests. The 5.0-mm steel system was significantly stiffer than 4.0-mm steel (lateral bending/torsion) and 4.75-mm titanium (torsion/flexion) constructs. Rod surface stress was significantly lower for the 4.75-mm titanium rod compared with 4.0-mm and 5.0-mm steel rods for all tests. CONCLUSIONS: The percentage of yield stress was lowest for the 4.75-mm Ti rod for all tests due to titanium's greater yield stress. This suggests the 4.75-mm rod has a lower fatigue failure risk than either steel construct.

Animals↗

PubMed2007

Abductor length alterations in hips with SCFE deformity.

Proximal femoral osteotomy may improve clinical outcomes in patients with residual deformity after slipped capital femoral epiphysis. Whether this procedure improves abductor mechanics is not well established. We hypothesized that abductor lengths would be shorter in patients with slipped capital femoral epiphysis compared with normal controls, and a femoral neck base osteotomy would create more normal abductor lengths than an osteotomy performed below the greater trochanter. Abductor muscle lengths were measured in normal, mild, and severe slipped capital femoral epiphyses sawbone models and after two methods of surgical correction. We observed decreases in abductor lengths in patients with slipped capital femoral epiphysis compared with normal controls when positioned in greater than 45 degrees flexion. There were fewer differences in abductor lengths after femoral neck base osteotomies than after subtrochanteric osteotomies. The femoral neck base osteotomy approximated normal abductor lengths more closely than the subtrochanteric osteotomy. The femoral neck base osteotomy restored the hip abductor relationship better than an osteotomy performed below the greater trochanter. We did not address the question of whether this improved relationship directly influenced function.

Adolescent↗

PubMed2006

Biomechanical comparison of the screw-bone interface: optimization of 1 and 2 screw constructs by varying screw diameter.

STUDY DESIGN: In vitro biomechanical investigation of 1 and 2-screw anterior scoliosis constructs with varying screw diameters. OBJECTIVE: To determine a possible optimal configuration of screw number and diameter at varying levels within the thoracic spine for anterior vertebral body fixation. SUMMARY OF BACKGROUND DATA: Single-rod systems are typical in anterior thoracic and thoracolumbar correction of adolescent idiopathic scoliosis; although dual rod systems may offer more flexural stability. Loss of fixation remains problematic, particularly in the proximal thoracic vertebrae, and it remains unclear how screw diameter or the number of screws within the vertebrae affect fixation. METHODS: Individual vertebral levels from 10 cadaveric thoracic spines were randomly assigned to either 1 or 2 screws of 5, 6, or 7-mm diameter. Bone-screw interface failures were created in coronal plane cantilever plow, and failure loads were compared across vertebral levels for each instrumentation method. RESULTS: Two-screw constructs had significantly higher failure loads than single-screw constructs, while increasing screw diameter also produced significant changes in fixation strength. Two-screws had improved performance in the mid and lower thoracic spine, while a single screw was more stable in the upper thoracic spine. CONCLUSIONS: Failure modes for 1-screw constructs almost entirely (89%) showed gradual plowing through the bone, whereas acute fracture through the vertebral body or pedicles were common forms of failure (85%) for 2-screw constructs.

Aged↗

PubMed2006

Biomechanical comparison of two different periarticular plating systems for stabilization of complex distal humerus fractures.

BACKGROUND: Complex intra-articular distal humerus fractures are relatively uncommon injuries but are fraught with poor outcomes such as malunion, elbow stiffness and deformity. Various types of internal fixation screw-plate constructs have been developed to improve fixation. Specifically, a 90 degrees offset periarticular system lowers the profile on the lateral epicondyle, yet it is unclear how this design compares to other plate constructs. This study compared the mechanical stiffness and plate surface strains between two types of constructs for stabilization of complex distal humerus fractures. METHODS: Identical bi-columnar segmental intra-articular fractures were created in ten epoxy composite left humeri. Models were randomly assigned to two groups (n=5/group) with either parallel plates or perpendicular plates. Rosette strain gages were placed at the most distal possible space on the lateral plate for both constructs. Models were mechanically tested with estimates of physiologic loads in flexion, extension, varus, valgus axial compression and axial torsion. Data for mechanical stiffness, transverse plate strain and longitudinal plate strain were compared with a one-way ANOVA (P<0.05). FINDINGS: There was no statistical difference in stiffness in any direction. The longitudinal strain for the 90 degrees construct was significantly lower in axial compression. The 180 degrees system demonstrated significantly lower transverse strains during axial torsion. INTERPRETATION: Both systems demonstrated similar mechanical stiffness theoretically providing similar fracture stabilization. Plate strain differences may affect fragment position, but it is unclear how much plate loading occurs in vivo. Surgeon experience and preference may dictate the choice of a plate construct for this fracture configuration.

Biomechanical Phenomena↗

PubMed2006

A biomechanical and radiographic analysis of standard and intracortical suture anchors for arthroscopic rotator cuff repair.

PURPOSE: To compare the fixation strength and radiographic motion of an anchor designed for intracortical (IC) fixation (FT Anchor, Arthrex, Naples, FL) with that of standard anchors used for rotator cuff repair. TYPE OF STUDY: In vitro human cadaveric biomechanical study. METHODS: Four types of metallic suture anchors (8 per group) were randomly inserted into human cadaveric humeri using an IC anchor and 3 types of standard anchors. Anchors were inserted 45 degrees to the humeral head surface and 90 degrees to the rotator cuff line of action. Anchors were tested under physiologic loads for 500 cycles followed by a failure test. The number of cycles, failure mode, and failure load were recorded. Fluoroscopy was used to measure rotation and displacement of the anchor within the humeral head during testing. Data were analyzed using a 1-way analysis of variance with a correction for multiple comparisons. RESULTS: There were no significant differences in anchor displacement or rotation measured by fluoroscopy after cyclic loading. Total construct displacement across anchors ranged from 4.9 to 7.8 mm, well beyond the 3-mm failure criterion reported in the literature. The IC anchor had a statistically significant greater failure load than the other devices. There was no significant difference in failure load between the other 3 anchors. The anchor had the greatest number of cycles to 3 mm of failure. This was not significantly different than the TwinFix anchor (Smith & Nephew, Andover, MA), but both values were significantly greater than both the Super Revo (Linvatec, Largo, FL) and Fastin RC (DePuy Mitek, Raynham, MA) anchors. CONCLUSIONS: Anchor motion accounted for about one third of total displacement of the suture/anchor construct. IC fixation anchors performed well compared with standard anchors in human cadaveric bone. CLINICAL RELEVANCE: Fluoroscopic imaging showed both rotation and displacement of the anchor within the humeral head which may contribute to early gap formation after rotator cuff repairs.

Aged↗

PubMed2006

Evaluation of 5 knots and 2 suture materials for arthroscopic rotator cuff repair: very strong sutures can still slip.

PURPOSE: To compare a standard suture material with a newer material using multiple arthroscopic knot configurations and to evaluate the biomechanical performance of a new sliding-locking knot compared with 4 surgical standards. TYPE OF STUDY: Controlled laboratory study. METHODS: Five knots were evaluated (Weston, Tennessee, Duncan, SMC, and the new San Diego knot) using 2 suture materials, No. 2 Ethibond (Ethicon, Somerville, NJ) or No. 2 Fiberwire (Arthrex, Naples, FL). Eight samples were tested for each knot-suture configuration. Samples were pretensioned to 10 N and then loaded from 10 to 45 N for 1,000 cycles. Intact knots were loaded to failure. RESULTS: Fiberwire had significantly higher load-to-failure (276 +/- 24 N) compared with Ethibond (111 +/- 13 N) (P < .001), although there was no significant difference as a function of knot configuration. Of the 40 Fiberwire knots, 3 failed by early slippage during cyclic loading and 8 slipped at very low tension during load-to-failure. None of the Ethibond knots and none of the San Diego knots failed by early slippage. CONCLUSIONS: Surface characteristics and suture construction affect the tendency for knot slippage. CLINICAL RELEVANCE: Surgeons should understand the impact of handling characteristics, frictional properties, and ultimate failure load when selecting suture materials and knots for arthroscopic repair.

Arthroscopy↗

PubMed2006

Biomechanical comparison of a novel percutaneous transfacet device and a traditional posterior system for single level fusion.

Posterior spinal fusions are indicated for a variety of spinal disorders. Transfacet fixation minimizes soft tissue disruption and preserves the adjacent facet joint. This technique is uncommon due to concerns with biomechanical stability and proper implant placement. For these reasons, a length adjustable implant may obviate the clinical concerns but necessitates biomechanical study. This study evaluated the in vitro biomechanical stability between a novel transfacet fixation device compared with standard pedicle screws during cyclic physiologic loading in a human cadaveric model. Cadaveric L4-L5 lumbar motion segments from 16 human spines were tested in cyclic flexion/extension, lateral bending, and torsion after insertion of either transfacet fixation devices or 5.5 mm pedicle screw instrumentation. A load cell was used to measure the compressive forces on the anterior column during testing. Motion segment stiffness and anterior column compression were analyzed with a 1-way analysis of variance (P<0.05). The transfacet device demonstrated a statistically similar stiffness when compared with the pedicle screw system for each test direction. For anterior column loading during physiologic testing, there were no biomechanical differences between stabilization systems. Percutaneous transfacet fixation is an attractive surgical option for single-level spinal fusions. A biomechanical evaluation of a novel device for this application demonstrated similar stability to a pedicle screw system. The length adjustability of the device may alleviate concerns for precise device placement and the biomechanical stability may produce similar rates and quality of posterior spinal fusions.

Aged↗

PubMed2005

Biomechanical comparison of lumbosacral fixation using Luque-Galveston and Colorado II sacropelvic fixation: advantage of using locked proximal fixation.

STUDY DESIGN: Biomechanical evaluation of sacropelvic fixation strategies as they apply to neuromuscular scoliosis. OBJECTIVES: The primary objective was to compare the rigidity of 2 methods of sacropelvic fixation (Galveston vs. Colorado II). The secondary objective was to evaluate the effect on construct rigidity by adding a pair of L1 pedicle screws to a Luque wire construct. SUMMARY OF BACKGROUND DATA: The Galveston modification to the Luque rodding system has become standard for treating childhood and adolescent neuromuscular scoliosis. The Galveston method provides reasonable lumbo-pelvic fixation with a relatively simple method of insertion. Clinical reviews of sagittal plane stability in neuromuscular patients with Galveston fixations performed at our institution have led to concerns regarding the technique's ability to maintain proper lumbar lordosis. This concern has generated interest in evaluating biomechanical stability of more rigid fixation methods in these long spino-pelvic constructs. As such, the following biomechanical study evaluated lumbosacral stability of 2 sacropelvic fixation methods: the standard Luque-Galveston method and the Colorado II sacropelvic fixation method using the Chopin plate-screw block. As a secondary interest, evaluations of the rigidity of the proximal construct when using pedicle screw fixation were completed. It was hypothesized that one additional point of rigid fixation at the thoracolumbar junction may make substantial improvement in rigidity to the otherwise Luque construct. METHODS: Lumbo-pelvic segments of human cadaveric specimens were instrumented with L1 pedicle screws, sublaminar wires between L2 and L5, and sacropelvic fixation with either Galveston rods or Colorado II sacropelvic plates using S1 screws, S2 alar screws, and iliac screws. Tests were conducted for physiologic flexion-extension and torsional loading. Construct stiffness between L1-S1 was determined for each specimen. Motion measurement data were collected between L1-L5 and L5-S1 using a noncontact marker system. Statistical analysis included a 2 -way analysis of variance (dependent variables: construct/locked screw) with the Tukey post hoc correction for multiple comparisons (P < 0.05). RESULTS: The flexion and extension bending stiffness of the construct was similar between the Galveston and Colorado II constructs. Both constructs were stiffer when the L1 screws were locked rigidly to the rod. Torsional stiffness followed similar trends with no significant difference between the systems, although more rigid in more cases when the L1 screws were locked to the rod. Regarding limiting L5-S1 motion during flexion and extension loading, the Colorado II construct did so to a higher degree. There was no difference in torsional motion between the 2 constructs. Locking the L1 pedicle screws reduced torsional motion but had no effect on flexion-extension motion at L5-S1. CONCLUSION: The 2 methods of sacropelvic fixation provided similar construct stiffness, although the Colorado II method had less L5-S1 motion on flexion-extension testing, and the Galveston construct tended (although not statistically) to be stiffer in torsional loading. The addition of a pair of L1 pedicle screws increased the construct stiffness for both constructs by approximately 50%.

Biomechanical Phenomena↗

PubMed2005

Failure mode of suture anchors as a function of insertion depth.

BACKGROUND: Surgeons can control not only the angle but also the depth of suture anchor placement during arthroscopic rotator cuff repair, although the tendency may be to place suture anchors on the deep side to avoid damage from prominent anchor eyelets. However, little information is available regarding possible effects of suture anchor depth on construct failure mechanisms. HYPOTHESIS: Anchor depth affects the mode of suture failure with physiologically relevant cyclic loads. STUDY DESIGN: Controlled laboratory study. METHODS: Metallic screw-in suture anchors loaded with No. 2 braided polyester sutures were inserted into the bovine infra-spinatus footprint with the eyelet proud, standard, or deep. Sutures were hand tied to create a closed loop. Constructs were cyclically loaded from 10 to 90 N and, if still intact at 500 cycles, taken to ultimate failure (maximum load). RESULTS: When clinical failure was defined as greater than 3-mm construct elongation, anchors placed with the eyelet deep experienced statistically earlier clinical failure via cutting of the suture through the bone (P < .02). However, anchors placed at this level did not experience catastrophic failure during cyclic loading. The standard and proud anchors experienced 3 mm of elongation at a greater number of cycles, but the suture material degraded at the anchor eyelet, and a majority of these constructs broke during cyclic physiologic loading. At failure testing, the deep anchors had a significantly increased failure load (164 N) compared to standard (133 N) (P < .04) and proud (113 N) anchors (P < .005). CONCLUSION: Varying the depth of suture anchor insertion changes the mechanical properties and mode of failure of suture anchor constructs. CLINICAL RELEVANCE: Surgeons should be aware of the effects of suture anchor depth and abrasive eyelet wear on construct failure during arthroscopic rotator cuff repair.

Animals↗

PubMed2005

Biomechanical comparison of bioabsorbable sutureless screw anchor versus suture anchor fixation for rotator cuff repair.

PURPOSE: Anchors that directly fix the rotator cuff to bone without using sutures may simplify the challenges associated with arthroscopic rotator cuff repair. This study compared a bioabsorbable screw fixation device to a standard suture anchor fixation method. TYPE OF STUDY: Randomized trial. METHODS: A poly-L-lactide acid (PLLA), bioabsorbable screw anchor was compared with a metallic suture anchor loaded with No. 2 braided polyester suture in 12-week-old fresh-frozen bovine shoulders. Twelve specimens were randomly assigned to the 2 repair groups (6 per group). A 1 x 2-cm defect was created at the insertion site of the infraspinatus tendon. Two anchors were implanted 1 cm apart in the anatomic insertional area of the infraspinatus tendon. After preconditioning to 10 N, each construct was cycled between 10 and 180 N for up to a maximum of 2,500 cycles at a rate of 33 mm/second using a materials testing machine. The number of cycles to 5- and 10-mm gap formation at the repair site and mode of failure were recorded. Clinically, 10-mm gap formation defines complete fixation failure. Data were evaluated using an analysis of variance with significance set at P < .05. RESULTS: Mattress suture fixation had significantly higher number of cycles to 10-mm failure compared with the PLLA device (P = .015). Failure occurred by tissue pullout in half the specimens and by device failure in the other half, without differences in failure modalities between groups. CONCLUSIONS: This anchor provides low-profile fixation of the rotator cuff and eliminates suture placement and knot tying during arthroscopic and mini-open cuff repair. However, these data suggest that this anchor may not perform adequately under cyclic loading conditions in patients during postoperative rehabilitation. CLINICAL RELEVANCE: This bioabsorbable anchor may not provide sufficient fixation for rotator cuff repair in humans.

Absorbable Implants↗

PubMed2005

Simulated hypergravity running increases skeletal and cardiovascular loads.

PURPOSE: Eight recreational and eight competitive athletes were studied to determine the cardiovascular and musculoskeletal effects of running in hypergravity conditions simulated by the use of lower body negative pressure (LBNP). Subjects are sealed in a LBNP chamber with the use of a flexible, neoprene waist seal and the chamber pressure is reduced to provide an increased vertical force. METHODS: Subjects ran on a treadmill at 1.0, 1.1, and 1.2 times body weight (BW) with increased loads provided by LBNP. Heart rate (HR), oxygen consumption (VO(2)), vertical ground reaction force (GRF), dynamic knee range of motion (ROM), and the electrical activities (EMG) of the tibialis anterior, medial gastrocnemius, vastus medialis obliquous, and biceps femoris muscles were measured. RESULTS: LBNP produced a significant increase in HR at the 1.1-BW and 1.2-BW levels in both recreational and competitive athletes when compared with the 1.0 BW condition. Both VO(2) and GRF were increased significantly at 1.2 BW. No significant changes were observed in knee ROM or peak EMG amplitude with LBNP in either recreational or competitive athletes. CONCLUSION: Increased HR and VO(2) indicate an increased cardiovascular load whereas increased GRF indicates an increased skeletal load. The lack of change in muscle activation and knee ROM point to a preservation in gait mechanics. Increased cardiovascular and skeletal loads with preservation of gait mechanics suggest that exercise within LBNP may be an effective training modality to improve athletic performance.

Adult↗

PubMed2004

[Changes in the lengths of the gluteus medius and gluteus minimus muscles with trochanteric transfer following pelvic support osteotomy: a biomechanical study].

OBJECTIVES: Using a synthetic bone model, we investigated changes in the muscle length of the gluteus medius and gluteus minimus following trochanteric osteotomy and pelvic support osteotomy (PSO) and compared the results with those of traditional PSO. METHODS: On two pelvises and four femurs, the lengths of the gluteus medius and gluteus minimus were measured in the following circumstances, with the hips in neutral position and in 45 degrees of flexion: (i) alignment of the hip joint with normal congruency; (ii) dislocated hip joint; (iii) following an osteotomy 2.5 cm below the lesser trochanter and stabilization with an angulation of 45 degrees of abduction; (iv) the insertion point was then moved 2 cm distally and 1 cm laterally, simulating a translation osteotomy of the greater trochanter. RESULTS: The muscle lengths increased with PSO compared to those of the dislocated hips (p<0.0001). Following distal and lateral translation osteotomy, the lengths significantly exceeded those obtained with traditional osteotomy (p<0.002), but were significantly less than those in the neutral position (p<0.001). Measurements in 45 degrees of flexion yielded similar results. Normal lengths could not be obtained in any of the procedures. CONCLUSION: Distal and lateral translation osteotomy following traditional PSO seems to increase the length of the abductor moment arm more than that obtained by traditional PSO alone.

Biomechanical Phenomena↗

PubMedDate not supplied

Biomechanical evaluation of kyphoplasty with calcium sulfate cement in a cadaveric osteoporotic vertebral compression fracture model.

BACKGROUND CONTEXT: Vertebral compression fractures can cause deformity, pain, and disability. Kyphoplasty involves percutaneous insertion of an inflatable balloon tamp into a fractured vertebra followed by injection of polymethylmethacrylate (PMMA) bone cement. PMMA has several disadvantages such as potential thermal necrosis and monomer toxicity. Calcium sulfate cement (CSC) is nontoxic, osteoconductive, and bioabsorbable. PURPOSE: To evaluate the biomechanical performance of CSC for kyphoplasty in cadaveric osteoporotic vertebral bodies. STUDY DESIGN: Destructive biomechanical tests using fresh cadaveric thoracolumbar vertebral bodies. METHODS: Thirty-three vertebral bodies (T9 to L4) from osteoporotic cadaveric spines were disarticulated, stripped of soft tissue, and measured for height and volume. Each vertebral body was compressed at 0.5 mm/s using a hinged plating system on a materials testing machine to create an anterior wedge fracture and reduce the anterior height by 25%. Pretreatment strength and stiffness were measured. Two KyphX inflatable balloon tamps were used to reexpand each vertebral body. After randomization, three groups were created: Group A-no cement; Group B-PMMA; Group C-calcium sulfate cement. Groups B and C were filled with the corresponding cement to 25% of the vertebral body volume. All vertebral bodies were then recompressed by 25% of the post-kyphoplasty anterior height to obtain posttreatment strength and stiffness. RESULTS: Treatment with PMMA restored vertebral strength to 127% of the intact level (4168.2 N+/-2288.7) and stiffness to 70% of the intact level (810.0 N/mm+/-380.6). Treatment with CSC restored strength to 108% of the intact level (3429.6 N+/-2440.7) and stiffness to 46% of the intact level (597.7 N/mm+/-317.5). CSC and PMMA were not significantly different for strength restoration (p=.4). Significantly greater strength restoration was obtained with either PMMA or CSC, compared with the control group (p=.003 and .03, respectively). Stiffness restoration tended to be greater with PMMA than for CSC, but this difference was not statistically significant (p=.1). Both cements had significantly greater stiffness when compared with the control group (p=.001 and p=.04, respectively). CONCLUSIONS: Use of CSC for kyphoplasty yields similar vertebral body strength and stiffness as compared with PMMA. It may be a useful alternative bone cement for kyphoplasty. Further studies are required to assess the bioabsorption of CSCs after kyphoplasty in vivo.

Aged↗

PubMedDate not supplied

Mechanical and structural characteristics of the new BONE-LOK cortical-cancellous internal fixation device.

The purpose of this study was to evaluate the structural and mechanical characteristics of a new and unique titanium cortical-cancellous helical compression anchor with BONE-LOK (Triage Medical, Inc., Irvine, CA) technology for compressive internal fixation of fractures and osteotomies. This device provides fixation through the use of a distal helical anchor and a proximal retentive collar that are united by an axially movable pin (U.S. and international patents issued and pending). The helical compression anchor (2.7-mm diameter) was compared with 3.0-mm diameter titanium cancellous screws (Synthes, Paoli, PA) for pullout strength and compression in 7# and 12# synthetic rigid polyurethane foam (simulated bone matrix), and for 3-point bending stiffness. The following results (mean +/- standard deviation) were obtained: foam block pullout strength in 12# foam: 2.7-mm helical compression anchor 70 +/- 2.0 N and 3.0-mm titanium cancellous screws 37 +/- 11 N; in 7# foam: 2.7-mm helical compression anchor 33 +/- 3 N and 3.0-mm titanium cancellous screws 31 +/- 12 N. Three-point bending stiffness, 2.7-mm helical compression anchor 988 +/- 68 N/mm and 3.0-mm titanium cancellous screws 845 +/- 88 N/mm. Compression strength testing in 12# foam: 2.7-mm helical compression anchor 70.8 +/- 4.8 N and 3.0-mm titanium cancellous screws 23.0 +/- 3.1 N, in 7# foam: 2.7-mm helical compression anchor 42.6 +/- 3.2 N and 3.0-mm titanium cancellous screws 10.4 +/- 0.9 N. Results showed greater pullout strength, 3-point bending stiffness, and compression strength for the 2.7-mm helical compression anchor as compared with the 3.0-mm titanium cancellous screws in these testing models. This difference represents a distinct advantage in the new device that warrants further in vivo testing.

Bone Screws↗

PubMedDate not supplied

Freedom of acetabular fragment rotation following three surgical techniques for correction of congenital deformities of the hip.

Femoral head coverage achieved with an acetabular osteotomy for hip dysplasia is achieved by acetabular rotation that can be restricted by osteotomy orientation and soft tissue attachments to the acetabular fragment. Procedures that allow excess rotation or motion in an undesirable direction (especially external rotation) may have undesirable consequences. Rotational aspects of these procedures have not been well described and are not well appreciated radiographically. This study examined the angular motion related to three operative techniques for redirectional acetabular osteotomies. The freedom of motion allowed for the Ganz periacetabular, Carlioz triple, and Tonnis triple osteotomies was quantified using three-dimensional motion measurement. The Ganz osteotomy allowed the greatest amount of motion. The Carlioz osteotomy allowed statistically less motion and depicted the "coupled motion" phenomenon in which a maximal angular rotation in one plane (abduction) is associated with a predicted angular change in another plane (external rotation). The Tonnis osteotomy allowed freedom of motion similar to the Ganz osteotomy without coupled motion.

Acetabulum↗

PubMedDate not supplied

A comparison of the fixation stability of multiple screw constructs for two types of pelvic osteotomies.

Periacetabular osteotomies are performed to improve femoral head coverage in a variety of clinical conditions. The stability of the osteotomy fragment between Ganz's periacetabular osteotomy was compared with that of Tonnis's triple innominate osteotomy under simulated weight-bearing conditions with a variety of screw constructs using three-dimensional analysis of fragment displacement and angular rotation. Generally, small amounts of displacement (<2 mm) and angular rotation (<3 degrees) were encountered for each fixation technique. Because the Ganz procedure violates the triradiate cartilage, it is not feasible in the immature patient. Thus, an understanding of fixation requirements in other available osteotomies to maintain fragment position is necessary.

Acetabulum↗

PubMedDate not supplied

Biomechanical analysis of single screw fixation for slipped capital femoral epiphysis: are more threads across the physis necessary for stability?

PURPOSE: To evaluate single screw fixation stability, in the treatment of slipped capital femoral epiphysis, as a function of screw thread distribution across the physis. STUDY DESIGN: In vitro biomechanical study. METHODS: Thirty porcine proximal femurs were sectioned through the physeal line and stabilized with a cannulated 7.3-mm stainless steel AO screw. The distal 16 mm of each screw was threaded (5 threads). The femurs were randomized into 5 groups (1, 2, 3, 4, or 5 threads across the physis) and biomechanically tested to determine failure load (N) and stiffness (N/mm). RESULTS: Femurs with 2 or 3 threads across the physis had a significantly greater load to failure and stiffness compared with femurs with 1, 4, or 5 threads across the physis (P < 0.05). CONCLUSIONS: Thread distribution across the physis seems to be important. When using screws with a 16-mm thread, greatest strength and stiffness are achieved when 40%-60% of threads engage the epiphysis, with a significant decrease when greater than 80% of threads cross the physis. Too few threads in the epiphysis as well as too few in the metaphysis both lead to decreased stability. CLINICAL RELEVANCE: This study challenges the belief that compression across the physis maximizes slipped capital femoral epiphysis fixation stability. We recommend equal distribution of threads across the physis when using 16-mm thread screws, and we postulate that screws with a greater thread length (32 mm or fully threaded) would increase fixation stability even further. Optimizing purchase may decrease the incidence of slip progression, especially as the prevalence of obesity increases in the adolescent population.

Animals↗