Mark Sartori

The effect of immediate constrained digital motion on the strength of flexor tendon repairs in chickens

Profundus tendon lacerations were repaired in the central toes of 216 chickens, and the digits were either immobilized in a cast or allowed immediate constrained motion in a tethering splint. The effect of digital motion on the early phases of tendon healing was investigated by comparing the rupture strengths of the two groups during the first 40 days after repair. The repairs in immobilized digits showed marked decreases in strength during the first 20 days, while the tendons in mobilized digits showed immediate and progressive gains in strength through the time intervals studied. By 5 days, the difference in strength between the two groups was significant (p less than 0.05), and the magnitude of this difference increased with time. The model developed in this study demonstrates that an initial loss of flexor tendon repair strength is not inevitable. Immediate constrained digital mobilization allows progressive tendon healing without an intervening phase of tendon softening.

Pedicle morphology of the immature thoracolumbar spine.

Study Design. Human vertebral morphologic data were compiled from anatomic skeletal collections from three museums. Objectives. To quantify the morphometric characteristics of the pedicles of the immature thoracolumbar spine. Summary of Background Data. Little is known of pedicle morphology of the immature spine as related to pedicle screw fixation. Methods. A total of 75 anatomic skeletal specimens were acquired from C1 to L5 in the age range of 3 to 19 years. The data were collected and analyzed using a computerized video analysis system. Each vertebral pedicle was measured in the axial and sagittal planes. The measurements included the minimum pedicle width, the pedicle angle, the distance to anterior cortex, and anteroposterior and interpedicular spinal canal diameters. Results Wide variation in pedicle morphology between specimens at each vertebral level was found in the young population. In general, compared with the average adult data, a younger spine demonstrated a near uniform reduction in the linear pedicle dimensions at each vertebral level. Pedicles from the lower lumbar vertebrae attained dimensions acceptable for standard screw sizes at an earlier age than in the thoracic vertebrae. Conclusions. The data in this study indicates that pedicle screws may be used in the adolescent spine. However, care should taken to accurately ascertain pedicle size before surgery so that improper use of screws can be avoided. Growth of the pedicles in relation to the spinal canal indicates that the increase in pedicle size is lateral to the spinal canal.

Acetabular micromotion as a measure of initial implant stability in primary hip arthroplasty

Micromotion has been shown to affect bony ingrowth into cementless components. This study was designed both to quantitate initial micromotion at the prosthesis-periacetabular bony interface and to compare different methods of commonly employed acetabular component fixations, ie, a press-fit hemispherical titanium cup, a press-fit hemispherical titanium cup with one and two dome screws, a press-fit titanium hemispherical cup with three spikes, and a cemented chromium-cobalt cup. The press-fit component without screws demonstrated the greatest motion equaling 162 microns at the ilium, 97 microns at the publis, and 54 microns at the ischium. With one and two screws placed into the dome, the mean ileal displacement decreased by 28 microns (17%) and 36 microns (22%), respectively. Dome screw placement demonstrated a minimal effect at the pubis and ischium. Compared to the press-fit component without augmentation, the tri-spike motion was less at the pubis and ischium. The cemented prosthesis provided the least amount of motion in all three areas tested. This experiment demonstrates that the ilium provides the least amount of support to immediate acetabular fixation, while the pubis (anterior column) and ischium (posterior column) provide more stability. One dome screw does not affect the stability of a hemispherical prosthetic cup significantly. A two dome screw fixation provides an added method of support at the ilium, but fails to decrease motion at the pubis or ischium significantly. The tri-spike fixation does not restrict motion at the ilium to the extent as the dome screws, but its effect at the ischium and pubis is much more pronounced. The obvious difference between initial motion seen with cemented versus uncemented components may suggest that before surgery, patients may need a period of protected weight bearing until ingrowth has occurred.

Instability of the lumbar burst fracture and limitations of transpedicular instrumentation

Study Design This study analyzed the changes in the load-displacement behavior of lumbar spine segments caused by burst fractures that were experimentally produced in fresh human cadaveric spines. The effect of three transpedicular surgical constructs on stability was investigated in each specimen. Objectives To quantify the loss of mechanical stiffness caused by the injury, and to evaluate the stiffness of three transpedicular surgical constructs. Summary of Background Data Although various investigators have studied the biomechanical characteristics of the burst fracture and surgical stabilization techniques, few have reported quantitative data on the three-dimensional biomechanical instability of these fractures. Methods Load-displacement data were acquired in flexion, lateral bending, and axial rotation for intact specimens, after the L1 burst fracture was created and after the T12-L2 segments were stabilized using Luque plates, VSP plates, and Isola rods with one transverse connector. Results Spines with burst fractures showed a bilinear load-displacement behavior with significant instability (loss of stiffness relative to intact) at low loads (up to 3 N.m) in flexion, lateral bending, and axial rotation. The loss of stiffness was greatest in axial rotation over the entire load range (up to 10 N.m). If posterior element injury also was present, a significantly larger loss of stiffness was obsarved in flexion and axial rotation. The three transpedicular constructs improved the stability of the injured spine beyond that of the intact spine in flexion and lateral bending at low loads. At high loads, they restored the stiffness to intact levels. However, in axial rotation they did not restore the stiffness to pre-injury level, particularly when the posterior column was disrupted. Conclusions Reduction of the burst fracture returns the spine to its position of greatest inherent instability, essentially requiring the transpedicular instrumentation to be load bearing. To enhance mechanical stability, it may be necessary to augment the transpedicular construct, particularly when the posterior column is disrupted.

The Effects of Binge Alcohol Exposure on Bone Resorption and Biomechanical and Structural Properties are Offset by Concurrent Bisphosphonate Treatment

BACKGROUND Chronic alcohol consumption reduces bone mass and strength, increasing fracture risk for alcohol abusers. Mechanisms underlying this vulnerability involve modulation of bone remodeling. Direct effects of alcohol on bone formation have been documented; those on bone resorption are less well studied. Skeletal effects of exposure to high blood alcohol concentrations (BACs) attained during binge drinking have not been studied. We examined the effects of repeated binge-like alcohol treatment on bone resorption, bone mineral density and vertebral compressive strength in adult male rats treated with the aminobisphosphonate, risedronate. METHODS A binge alcohol exposure model was developed using intraperitoneal (IP) injection to administer a 20% (vol/vol) alcohol/saline solution (3 g/kg, 1X/day) on four consecutive days for 1, 2 or 3 weeks in 400 g rats, with and without weekly risedronate treatment (0.5 mg/kg, 1X/week). Total serum deoxypyridinoline (Dpd) a crosslink of bone type collagen released during resorption was measured by ELISA. Bone mineral density (BMD) was measured using peripheral quantitative computed tomography (pQCT). Vertebral compressive strength was determined using an Instron materials testing machine. Trabecular integrity was analyzed by computer-aided trabecular analysis system (TAS). RESULTS Peak BACs averaged 308.5 +/- 12 mg/dL; average BAC was 258.6 +/- 28.7 mg/dL at time of euthanasia. No significant effects of treatment were observed after 1 or 2 weeks of binge alcohol exposure. At 3 weeks of alcohol treatment serum Dpd was significantly increased (205%, p < 0.05) over controls. Bone mineral density (BMD) in cancellous bone of distal femur and lumbar spine were significantly decreased (34% and 21% respectively, p < 0.01) after 3 weeks of binge treatment. Vertebral (L4) compressive strength (maximum load sustained before failure) also decreased (27%, p < 0.05) after 3 binge alcohol cycles. Risedronate maintained the Dpd level (p < 0.01), BMD (p < 0.001) and vertebral structural biomechanical properties (p < 0.01) of binge-treated rats at control levels (E vs ER). Indices of trabecular architectural integrity [Trabecular bone volume/tissue volume (BV/TV), bone area (BAR) and trabecular separation (Tb.Sp)] analyzed at week 3 showed (BV/TV) and (BAR) were significantly reduced in alcohol-binged rats (p < 0.01), while (Tb.Sp) was significantly increased (p < 0.01). Risedronate also maintained the trabecular architectural indices of binge-treated rats at control levels (E versus ER, p < 0.01). CONCLUSIONS In adult male rats, BACs reflective of those attained during alcoholic binge drinking may affect the skeleton in part by stimulating bone resorption, an effect mitigated by risedronate.

Does screw configuration affect subtrochanteric fracture after femoral neck fixation

A subtrochanteric femur fracture after cannulated screw fixation of a femoral neck fracture is a devastating complication. We hypothesized that an apex-distal screw orientation would tolerate higher loads to subtrochanteric fracture. Human cadaveric femora were instrumented with three cannulated screws in either an apex-distal or an apex-proximal configuration. Specimens were loaded along the mechanical axis to failure creating a subtrochanteric femur fracture. Ultimate load to failure and the effect of bone density on load to failure were compared between groups. There was a greater load to failure in the apex-distal group compared with the apex-proximal group. The mean force to fracture in the apex-distal group (11,330 N; standard deviation = 3151 N) was greater than the mean force to fracture in the apex-proximal group (7795 N; standard deviation = 3194 N). Previous investigations have shown improved femoral neck fixation with an apex-distal configuration, but none has examined the relationship between screw orientation and subtrochanteric fractures. Our observations support the use of an apex-distal configuration for cannulated screw fixation of femoral neck fractures.

Effects of Growth Hormone Transgene Expression on Vertebrae in a Mouse Model of Osteogenesis Imperfecta

Study Design. A human growth hormone transgene was bred into mice of the Cola2oim (oim) lineage. Caudal (tail) vertebrae from male and female mice at early skeletal maturity and at midlife were evaluated for physical and biomechanical properties. Objective. To test whether constant low-level growth hormone expression within the marrow could improve structural or material properties of caudal vertebrae in oim mice. Summary of Background Data. A spontaneous genetic defect in a type I procollagen gene created the oim mouse model for osteogenesis imperfecta. Bones of heterozygous oim mice are biomechanically inferior to wild-type controls. Bone marrow expression of human growth hormone was demonstrated previously to enhance bone deposition and structural biomechanical properties in caudal vertebrae of transgenic mice. Methods. Compression tests were performed individually on three caudal vertebrae (Ca4, 5, and 6) from each mouse to determine their structural biomechanical properties. Volumetric and mineral content measurements were also made. In a subset of vertebrae, the ashing measurements were confirmed and extended by peripheral quantitative tomographic scanning, which also allowed calculation of the failure stress. Results. Heterozygous oim mouse vertebrae had structural and material properties inferior to the wild-type controls. Growth hormone transgene expression increased the size and mineral content of the vertebrae from mutant mice, and increased biomechanical structural values for maximum load and energy to failure. Failure stress was not improved. Conclusions. Growth hormone stimulation of size and bone mineral content of osteogenesis imperfecta mutant mouse caudal vertebrae contributed to their improved performance in axial compression. There was no evidence for improved material properties, however.

Biomechanical Comparison of the Milwaukee Brace (CTLSO) and the TLSO for Treatment of Idiopathic Scoliosis

The decision to choose between a CTLSO (Milwaukee brace) and a TLSO (such as the Boston system, Wilmington, Miami or Rosenberger orthosis) is affected by several factors including cosmesis, geographical preference and popularity of a given orthosis. Performance usually is a secondary consideration since objective comparisons have been difficult. A finite element model was used to quantify and compare the effects of the CTLSO and TLSO in increasing spinal stability as measured in terms of the critical load of right primary thoracic with left lumbar compensatory and left primary lumbar with right compensatory idiopathic scoliotic curves. While the CTLSO and TLSO were equal in stabilizing lumbar primary curves, the TLSO was 25 percent less effective in stabilizing primary thoracic curves. Pad and counterforce placement were critical factors influencing stability for both the CTLSO and TLSO, demonstrating the importance of proper initial fit and timely growth adjustments to ensure proper pad placement throughout the duration of wear.