Robert D. Welch

Effect of Recombinant Human Bone Morphogenetic Protein‐2 on Fracture Healing in a Goat Tibial Fracture Model

Bone morphogenetic proteins (BMPs) are considered to have important regulatory roles in skeletal embryogenesis and bone healing. Recombinant human BMPs (rhBMPs) have been shown to heal critical size defects and promote spinal fusion. We studied the effects of rhBMP‐2 in an absorbable collagen sponge (ACS) on bone healing in a large animal tibial fracture model. Bilateral closed tibial fractures were created in 16 skeletally mature goats and reduced and stabilized using external fixation. In each animal, one tibia received the study device (0.86 mg of rhBMP‐2/ACS or buffer/ACS), and the contralateral fracture served as control. The device was implanted as a folded onlay or wrapped circumferentially around the fracture. Six weeks following fracture, the animals were sacrificed and the tibiae harvested for torsional testing and histomorphologic evaluation. Radiographs indicated increased callus at 3 weeks in the rhBMP‐2/ACS treated tibiae. At 6 weeks, the rhBMP‐2/ACS wrapped fractures had superior radiographic healing scores compared with buffer groups and controls. The rhBMP‐2/ACS produced a significant increase in torsional toughness (p = 0.02), and trends of increased torsional strength and stiffness (p = 0.09) compared with fracture controls. The device placed in a wrapped fashion around the fracture produced significantly tougher callus (p = 0.02) compared with the onlay application. Total callus new bone volume was significantly increased (p = 0.02) in the rhBMP‐2/ACS fractures compared with buffer groups and controls regardless of the method of device application. The rhBMP‐2/ACS did not alter the timing of onset of periosteal/endosteal callus formation compared with controls. Neither the mineral apposition rates nor bone formation rates were affected by rhBMP‐2/ACS treatment. The increased callus volume associated with rhBMP‐2 treatment produced only moderate increases in strength and stiffness.

Experimental Tibial Plateau Fractures Augmented With Calcium Phosphate Cement Or Autologous Bone Graft

Background: Depression fractures of the tibial plateau are often managed with use of internal fixation and autologous bone-grafting to maintain an anatomical reduction. Bone-grafting, however, provides only limited stability. As calcium phosphate cements have appropriate mechanical properties, they may provide a more suitable alternative. The objective of this study was to compare the effect of a calcium phosphate cement with that of impacted cancellous autograft for maintaining an anatomical reduction in an experimental model of a tibial plateau fracture.Methods: Standardized cylindrical subchondral defects that were 8 mm in diameter and 10 mm deep were created bilaterally beneath the subchondral bone of the articular cartilage in the lateral tibial plateau of goats. An osteotome was used to fracture the overlying subchondral plate and articular cartilage. The plateau fracture fragment was completely depressed into the subchondral defect and then was anatomically reduced. The defects were randomly filled with either calcium phosphate cement or cancellous autograft. No internal fixation was used. The tibiae were harvested at varying time-periods that ranged from twenty-four hours to eighteen months. The stiffness of the healing augmented plateau fractures was determined. Histological specimens were assigned a score for degenerative changes. Loss of anatomic reduction was demonstrated in photomicrographs, and the amount of subsidence of the osteochondral fragment was measured in whole-mount histological sections.Results: The prevalence and degree of fracture subsidence was significantly reduced at all time-points in the defects treated with calcium phosphate cement compared with those filled with autograft (p < 0.05). There were no significant differences in fracture stiffness between the two treatment groups at any of the time-points examined. The calcium phosphate cement was rapidly resorbed, and the volume fraction of the calcium phosphate cement was decreased to 4% at six months. The trabecular bone volume in the defects was restored to that of the intact controls at six months in both treatment groups.Conclusions: Cancellous autograft did not maintain an anatomical reduction of the tibial plateau fractures in this model. In contrast, augmentation with calcium phosphate cement prevented subsidence of the fracture fragment and maintained articular congruency as the fracture healed. The improved articular congruency reduced the prevalence and severity of degenerative changes in the joint.Clinical Relevance: Reconstruction of intra-articular depression fractures of the tibial plateau is hampered by the loss of subchondral bone and the small fragment size. The use of calcium phosphate cement as a buttress in articular depression fractures may reduce the need for extensive internal fixation and allow earlier functional rehabilitation.

Histomorphometry of Distraction Osteogenesis in a Caprine Tibial Lengthening Model

Standardized histomorphometry of bone formation and remodeling during distraction osteogenesis (DO) has not been well characterized. Increasing the rhythm or number of incremental lengthenings performed per day is reported to enhance bone formation during limb lengthening. In 17 skeletally immature goats, unilateral tibial lengthenings to 20 or 30% of original length were performed at a rate of 0.75 mm/day and rhythms of 1, 4, or 720 times per day using standard Ilizarov external fixation and an autodistractor system. Two additional animals underwent frame application and osteotomy without lengthening and served as osteotomy healing controls. Histomorphometric indices were measured at predetermined regions from undecalcified tibial specimens. Within the distraction region, bone formation and remodeling activity were location dependent. Intramembranous bone formed linearly oriented columns of interconnecting trabecular plates of woven and lamellar type bone. Total new bone volume and bone formation indices were significantly increased within the distraction and osteotomy callus regions (Tb.BV/TV, 226% [p < 0.05]; BFR/BS, 235–650% [p < 0.01]) respectively, compared with control metaphyseal bone. Bone formation indices were greatest adjacent to the mineralization zones at the center of the distraction gap; mineral apposition rate 96% (p < 0.01); mineralized bone surfaces 277% [p < 0.001]); osteoblast surfaces 359% [p < 0.001]); and bone formation rate (650% [p < 0.01]). There was no significant difference (p < 0.14; R = 0.4) in the bone formation rate of the distracted callus compared with the osteotomy control callus. Within the original cortices of the lengthened tibiae, bone remodeling indices were significantly increased compared with osteotomy controls; activation frequency (200% [p < 0.05]); osteoclast surfaces (295% [p < 0.01]); erosion period (75%); porosity (240% [p < 0.001]). Neither the rhythm of distraction nor the percent lengthening appeared to significantly influence any morphometric parameter evaluated. Distraction osteogenesis shares many features of normal fracture gap healing. The enhanced bone formation and remodeling appeared to result more from increased recruitment and activation of bone forming and resorbing cells rather than from an increased level of individual cellular activity.

Bone changes associated with intraosseous hypertension in the caprine tibia.

We investigated the effects of increased intraosseous pressure on new-bone formation in the proximal metaphysis of the caprine tibia. Intraosseous hypertension was produced by obstruction of venous outflow by ligation of the popliteal vein draining the proximal aspect of the tibia and occlusion of the medullary space with bone cement (groups 1 and 2). After the obstruction of venous outflow (day 0), the intraosseous pressure measured at the proximal tibial metaphysis increased significantly from a mean of 15.5 millimeters of mercury before the obstruction to a mean of 28.7 millimeters of mercury in groups 1 and 2. In group 1, obstruction of venous outflow was combined with intraosseous infusion of autogenous whole blood under pressure to maintain the intraosseous pressure between thirty and forty-five millimeters of mercury during days 0 through 5. The time for venous drainage was still prolonged and intraosseous pressures were still increased on days 5 and 10 (means, 26.8 and 26.2 millimeters of mercury, respectively) in groups 1 and 2. The intraosseous hypertension produced in group 1 was associated with a significant increase in periosteal (138 per cent), endocortical (369 per cent), and cancellous new-bone formation (889 per cent) at the tibial metaphysis compared with control values. Osseous necrosis within the metaphysis was not observed.

Subchondral defects in caprine femora augmented with in situ setting hydroxyapatite cement, polymethylmethacrylate, or autogenous bone graft: biomechanical and histomorphological analysis after two-years

Juxta‐articular defects pose significant challenges due to the high risk of fracture of the subchondral plate and articular cartilage. We evaluated the mechanical and histomorphological repair process of caprine subchondral femoral defects augmented with either a bioresorbable in situ setting hydroxyapatite cement (HAC), polymethylmethacrylate (PMMA), autogenous bone graft (AG), or left empty. Twelve‐mm subchondral defects were made bilaterally in the medial femoral condyles of skeletally mature goats and augmented with a test material or left empty. Femurs were harvested at varying time periods out to 2 years and evaluated for subchondral stiffness and histomorphological indices. Several defects augmented using autograft or left empty sustained focal fracture of the subchondral plate. No HAC or PMMA augmented defects showed evidence of subchondral fracture. The HAC and PMMA augmented defects showed comparable stiffness at all time points. The mean volume fraction of HAC remaining within the defects progressively decreased from 96% at 24 h to 38% at 2 years. The new bone replacing the HAC appeared to have normal physiological architecture and orientation. In situ setting hydroxyapatite cement may be a viable alternative for the repair of subchondral defects with an important advantage that while undergoing gradual resorption and replacement with host bone, mechanical integrity of the skeletal defect is maintained.

The effect of the amount of limb lengthening on skeletal muscle.

The adaptation of tibialis anterior muscles after 20% and 30% gradual limb lengthening was evaluated. Eight skeletally mature neutered male goats had 20% (n = 4) or 30% (n = 4) tibial distraction at a rate of 0.25 mm three times per day. Muscles from lengthened and contralateral control limbs were harvested on completion of distraction. Fiber length and sarcomere length were measured followed by calculation of sarcomere number and muscle fiber-to-bone lengthening ratio. Fiber length and sarcomere number after 20% and 30% limb lengthening were significantly greater in the distracted muscles, whereas no difference in sarcomere length was detected. The difference in muscle fiber length and sarcomere number between distracted and control limbs was greater in the 30% than in the 20% group. The disproportion between the amounts of muscle fiber and bone length increase was similar after 20% and 30% lengthening. The results show that muscular adaptation continues during 20% to 30% limb lengthening by increasing fiber length. It seems that this increase occurs through serial sarcomere addition rather than sarcomere length alteration. The higher rate of musclerelated clinical complications after limb lengthening beyond 20% does not seem to be related to a failure of muscle fiber contractile elements to adapt to increasing limb length.

Effect of rhythm and level of distraction on muscle structure: an animal study.

To determine the optimal rhythm of distraction and level of osteotomy for maximal muscle preservation during limb lengthening, histologic and ultrastructural analysis of the peroneus tertius muscle was performed in 46 goats. Thirty-nine animals with 20% or 30% tibial lengthening at the rate of 0.75 mm per day at each osteotomy level were divided in two groups. The first group included 15 animals undergoing tibial middiaphyseal lengthening with automated distractor producing one (n = 5), four (n = 5), or 720 (n = 5) increments per day. The second group included 24 animals in which lengthening was performed at the proximal (n = 4), middle (n = 6), distal (n = 6), or double (n = 8) level of the tibia with distraction rhythm of three times per day. The remaining seven animals served as controls. Histomorphologic features of the lengthened muscles were characterized by a polymorphous appearance with foci of necrosis, atrophy, and sclerosis scattered among apparently normal muscle fibers. The intensity and dispersion of degenerative changes were in direct proportion to the amount of lengthening but in reverse proportion to the frequency of distraction. Tibial lengthening at the diaphysis caused intense fibroblast proliferation and remarkable muscle sclerosis, whereas metaphyseal distraction was associated with a predominance of muscle tissue regeneration over atrophy and sclerosis. Bifocal lengthening and a more fractionated rhythm of distraction were associated with enhanced preservation of existing muscle fibers and greater regenerative activity of the muscle.

Recombinant human bone morphogenic protein-2-enhanced anterior spine fusion without bone encroachment into the spinal canal : A histomorphometric study in a thoracoscopically instrumented porcine model

Study Design. A thoracoscopically assisted 5-level anterior spinal fusion and instrumentation model analyzing new bone formation when using recombinant human bone morphogenic protein-2 (rhBMP-2) with a collagen hydroxyapatite-tricalcium phosphate (HA/TCP) composite sponge carrier. Objective. To determine whether new bone formation extends beyond the posterior confines of the vertebral body encroaching into the spinal canal when rhBMP-2 is used to enhance anterior fusion. Summary of Background Data. A possible concern regarding the use of rhBMP-2 to enhance spinal fusion is the risk of unwanted bone formation leading to inadvertent fusion of adjacent levels or compression of neural elements. The safety of rhBMP-2 in one spinal application does not ensure similar results in other applications. Therefore, the expanded use of rhBMP-2 should occur only after carefully monitored preclinical and clinical studies for each new application. Methods. Eighteen pigs underwent thoracoscopically-assisted instrumentation and fusion of 5 contiguous levels (T5–T10) and randomly assigned to 4 treatment groups: group 1 (n = 6): rh-BMP-2 on a HA/TCP-collagen sponge (Medtronic Sofamor Danek, Memphis, TN); group 2 (n = 4): iliac crest autograft; group 3 (n = 4): empty; group 4 (n = 4): HA/TCP-collagen sponge (Medtronic Sofamor Danek) only. In groups 1 and 4, the HA/TCP collagen sponge was morselized into small granules and pushed through a bone delivery funnel for implantation into the disc. At 4 months after surgery, spines were sectioned longitudinally through the midsagittal plane and underwent undecalcified processing. Bone formation extending beyond the margins of the original discectomy and the confines of vertebral body were evaluated histomorphometrically at each operative level. Results. Recombinant human bone morphogenic protein-2 on a HA/TCP-collagen sponge induced significant new bone formation extending anterior to the confines of the vertebral body compared with the other treatment groups (P < 0.05). In addition, rhBMP-2 on a HA/TCP-collagen sponge induced significant new bone formation extending posterior to the original margins of the discectomy (P < 0.05). However, there was no new bone formation beyond the confines of the posterior vertebral body. The total bone volume in the rhBMP-2-HA/TCP-collagen sponge group was significantly greater compared with all other groups in both the discectomy fusion area and beyond the discectomy area (P < 0.05). Conclusions. Recombinant human bone morphogenic protein-2 on a HA/TCP-collagen sponge enhanced anterior spinal fusion and induced significant new bone formation extending beyond the margins of the original discectomy and anterior vertebral body, most likely secondary to migration of some morselized carrier fragments from the disc space. However, the new bone formation did not extend beyond the posterior confines of the vertebral body to encroach into the spinal canal because of the intact posterior anulus and/or posterior longitudinal ligament.

Successful treatment of unicameral bone cyst by single percutaneous injection of α-BSM

Background Unicameral bone cyst (UBC) is a benign bone lesion, recognized for its high rate of recurrence and need for repeat procedures to achieve healing. We hypothesized that the osteoconductive material apatitic calcium phosphate (α-BSM) could be effective in filling and stimulating resolution of UBC. The purpose of this study was to evaluate clinical and radiographic outcomes of UBC treated by a single injection of α-BSM. Methods Thirteen patients (6 male, 7 female) with a mean age of 10.5 years, underwent single percutaneous injection of α-BSM for presumed UBC. The aspiration of the cysts was followed by vigorous saline lavage using 2 wide bore needles to disrupt the cyst walls. α-BSM “paste” was then injected under fluoroscopic guidance. Radiographs were digitized to measure cystic area (millimeter squares) on 2 orthogonal views. Healing was rated according to a modified Neer outcome grading system. Nine of the 13 patients had had pathologic fractures in the past. Eleven of the 13 patients had had past unsuccessful treatment: multiple steroid injections in 6, curettage and bone grafting in 3, and bone marrow and demineralized bone matrix (Grafton) injection in 2. Results Five cysts were grade 1 (healed 100%), 6 grade 2 (healed >50%), 2 grade 3 (healed <50% with increased cortical thickness), and none grade 4 (recurrence/enlargement). The average resolution of cystic area in 11/13 cysts was 85.7% at final follow-up of 35.8 months (P=0.0001) with 2.8 mm of average gain in cortical thickness (P=0.0018). None of the 13 lesions required an additional procedure or repeat injection. All patients were clinically asymptomatic at latest follow-up. Conclusions This is the first study quantifying cyst resolution objectively according to actual decrease in area (millimeter squares). A single injection of α-BSM is a safe, minimally invasive and efficacious method to treat UBC in the pediatric population. Level of Evidence Case series, level IV

Effect of spinal construct stiffness on short segment fusion mass incorporation

Study Design Three segment (L3-L5) pedicle screw constructs were implanted in caprine spines, and the resulting ankylosis evaluated mechanically and compared 12 weeks after surgery. Objectives To determine if a construct of maximal stiffness could impair the biologic process of spinal arthrodesis by “stress-shielding.” Summary of Background Data Fusion mass stiffness is believed to be enhanced by increasing construct stiffness, although previous studies have used semirigid, nonconstrained constructs, which lose stiffness through cyclical loading. Device-related osteoporosis, reported to occur with stiff, constrained implants, may be more related to the presence of fusion induced by the implants rather than the implants themselves. Methods In 15 goats, L3-L5 segments were instrumented with pedicle screws, and four different diameters of rods (3.2 cm, 4.8 cm, 6.4 mm, and no rods) were implanted as longitudinal connections to vary the stiffness of the constructs. After 12 weeks, animals were killed and the segments were tested to determine their stiffness. Results In lateral bending, spines “fused” with rods (any size) were significantly stiffer (P = 0.03) than nonrodded spines. There was a trend toward stiffer segments with larger rods (4.8 cm or 6.4 mm) compared with 3.2 mm or no rods. There was a highly significant (P < 0.0001) increase in stiffness of all operated (rodded or nonrodded) segments compared with unoperated controls. Conclusions The enhancement of segmental stiffness by stiffer constructs was confirmed, suggesting a beneficial effect on spinal arthodesis by increasing stiffness, Stress shielding could not be shown.