John M Clark

University of Washington Seattle, Seattle, WA, United States

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Publications (7)18.97 Total impact

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    ABSTRACT: Type I collagen fibrils in tendons and ligaments assume a sinusoidal wave shape, or crimp, which straightens only with tensile load. The load response of crimp has been studied primarily in isolated subunits and not in complex, intact structures. The purpose of our study was to determine if freeze substitution fixation of an entire ligament could preserve changes in crimp morphology induced by functionally relevant loading conditions. We hypothesized that, in ligaments prepared by freeze-substitution fixation under load, crimp would progressively extinguish with increasing loads, and nonuniform strain following partial section could be detected from crimp morphology. Tensile loads ranging from 0 to 220 N were applied to patellar ligaments of 16 fresh rabbit stifle joints using simulated isometric quadriceps pull through the patella. The loaded joints were flash frozen with isopentane cooled in liquid nitrogen, then fixed using freeze substitution. Another six ligaments were loaded to 150 N following incision of the anterior third and evaluated under polarized light microscopy for crimp distribution. Ligaments with no or low loads could be identified by the presence of crimp on mid-sagittal sections. Strain distribution was inhomogeneous, in that the ligament displayed a consistent pattern of collagen fiber recruitment among three morphologically distinct bands seen on coronal sections. At very low loads (about 18 N), the fibers in a central band were uncrimped; anterior and deep bands uncrimped at higher loads. The crimp in the entire specimen was extinguished at about 67 N, which correlates closely with the previously reported toe-region of the stress-strain curve of the rabbit patellar ligament. When the anterior third was transected, fibers within that segment retained a crimp in ligaments prepared under loads that ordinarily would ablate all crimp. These findings suggest that freeze fixation could be used to map the functional microstructure of ligaments or tendons.
    Journal of Orthopaedic Research 05/2006; 24(4):793-9. · 2.88 Impact Factor
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    ABSTRACT: Regeneration of mammalian digit tips is well described; however, associated cellular or molecular events have not been studied in humans. We describe an in vitro human fetal model of response to digit tip amputation, and report expression of the transcription repressor Msx1 in the developing and regrowing human digit tip. Human fetal digits from specimens ranging from 53 to 117 days' estimated gestational age (EGA) were cultured in a defined serum-free medium with supplemented oxygen for time periods from 4 days to 4 weeks. Histology and immunohistochemistry were performed on paired control and tip-amputated digits. Regrowing tissue covered the cut end of the distal phalanx in digits up to 80 days' EGA. Msx1 expression was detected beneath the nail field in control digits to at least 70 days' EGA and at the regrowing tip of 57-day digits at 4 and 7 days post-amputation. Our results show that human fetal digits regrow tissue in vitro in response to tip amputation. This process appears spatially associated with Msx1 expression. Msx1 expression appears increased at the regrowing tip of 57-day digits by 4 days after amputation.
    Wound Repair and Regeneration 01/2006; 14(4):398-404. · 2.76 Impact Factor
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    ABSTRACT: We tested the hypothesis that articular cartilage adjacent to experimental osteochondral defects is not subject to unusual strains under load. A 2.5-mm drill hole was made in the medial femoral condyle of 15 knees from 10 adult rabbits. Experimental joints were loaded with simulated quadriceps force, then frozen under load and preserved by freeze-substitution fixation. Deformation in the region of the defect was evaluated by scanning electron and light microscopy and compared with nondrilled and nonloaded control knees. To simulate blood clot, alginate was placed into some defects before loading. In loaded knees, articular cartilage at the edge of the drill hole was abnormally flattened and folded into the defect. Opposing tibial cartilage or meniscus intruded into the femoral defect beyond the cement line. Alginate did not prevent incursion of opposing cartilage. In this standard drill-hole model, the articular cartilage defect is occupied by the opposing surface when a joint is loaded. Any tissue growing or surgically implanted in the defect is subject to loading and displacement, therefore complicating attempts to characterize the healing or regenerative potential in similar drill-hole models. Deformation of cartilage at the defect edge suggests load concentration or increased compliance. Either phenomenon would contribute to subsequent degeneration of the cartilage adjacent to defects.
    Clinical Orthopaedics and Related Research 02/2005; · 2.79 Impact Factor
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    ABSTRACT: This study characterizes the healing response of the glenoid after spherical reaming and prosthetic humeral head replacement in a canine model of glenohumeral hemiarthroplasty. The right glenoid of twelve skeletally mature female dogs was reamed to a uniform radius of curvature, removing all cartilage down to bleeding subchondral bone. The glenoid was not resurfaced. The humeral head was replaced with a stemmed metal prosthesis. Post-surgery, the operated limbs were immobilized for seven days, with motion allowed ad libitum thereafter. Fluorescent bone labels were administered to identify bone formation. These procedures were not complicated by instability, infection or death. Six animals were euthanized at 10 week and six more at 24 week. The intact glenohumeral joints were evaluated by gross examination, assessment of glenoid concavity, and light microscopy of methylmethacrylate sections. At 10 week, vascular fibrous tissue partially covered the glenoid, maintaining a concave surface congruent with the prosthetic humeral head. New bone formed at the margin of the glenoid, and the density of the periarticular trabecular bone increased. At 24 week, the healing was more advanced; thick fibrocartilaginous tissue covered the entire glenoid surface. These results demonstrate that spherical glenoid reaming produced a consistent healing response characterized by remodelling of the reamed bony concavity to a congruent, living, smooth, securely attached interface articulating with the humeral prosthesis.
    Journal of Orthopaedic Research 02/2005; 23(1):18-26. · 2.88 Impact Factor
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    ABSTRACT: Exact reconstruction of an osteochondral defect by autogenous transplantation (mosaicplasty) is difficult given the variation in joint surface contour. Clinical and experimental studies do not show the extent to which incongruity can be tolerated in autografting. Grafted articular cartilage will hypertrophy to correct the incongruity created by recession of the transplanted surface. Controlled laboratory study. To test the response of grafts to incongruities, osteochondral autografts were transplanted from the trochlea to the femoral condyle in adult male sheep stifle joints. In groups of 6 animals, graft surfaces were placed flush, countersunk 1 mm or countersunk 2 mm, then histologically analyzed 6 weeks after surgery. Cartilage thickness, condition of the articular surfaces, and preservation of hyaline characteristics were the primary features compared. Bony union, vascularization, and new bone formation were present in all grafts. Cartilage-to-cartilage healing did not occur. In flush specimens, cartilage changed minimally in thickness and histologic architecture. The specimens countersunk 1 mm demonstrated significant cartilage thickening (54.7% increase, P <.05). Chondrocyte hyperplasia, tidemark advancement, and vascular invasion occurred at the chondroosseous junction, and the surface remained smooth. Cartilage necrosis and fibrous overgrowth were observed in all grafts countersunk 2 mm. Minimally countersunk autografts possess a capacity for remodeling that can correct initial incongruities while preserving hyaline characteristics. Grafts placed deeper do not restore the contour or composition of the original articular surface. If preservation of normal hyaline cartilage is the objective, thin grafted articular cartilage can remodel, but the tolerance for incongruity is limited and probably less than that reported for an intra-articular fracture.
    The American Journal of Sports Medicine 01/2005; 32(8):1842-8. · 4.44 Impact Factor
  • The Journal of Bone and Joint Surgery 12/2003; 85-A(11):2249-51; author reply 2251. · 3.23 Impact Factor