Biomechanics of osteochondral graft insertion : : cartilage damage and protection strategies

Osteochondral graft repair is one of the most effective surgical treatments for focal cartilage defects. During such surgery, the load applied on the graft during impact insertion can result in damage to its articular cartilage, and therefore be detrimental to long-term effectiveness and clinical outcome. The exact mechanical stimuli by which impact loading causes such damage are unknown, with suggested and known mechanobiological factors and mediators including compressive stress, compressive strain, impulse, and energy density. Three hypotheses regarding osteochondral graft (OCG) insertion were tested in the dissertation. (1) Energy delivered to the graft is the critical biomechanical determinant of cartilage damage during impact. (2) Increasing tightness of graft-host fit leads to higher insertion energy and resultant cartilage damage. (3) Modifying the geometry of the graft can alter the mechanics of impact insertion and therefore provides cartilage protection by reducing energy delivery to the graft. Osteochondral sample (OCS) or osteochondral graft (OCG), as well as osteochondral recipient site (OCR), were harvested fresh from distal femora of adult bovine. An instrumented drop tower apparatus was used to apply a range of energy and to quantify energy delivered to samples, as well as a variety of other mechanical factors, during impact of OCS, or insertion of OCG into OCR. Damage to the articular cartilage was quantified as total crack length and viability of chondrocytes at the articular surface. During OCS impact and OCG insertion into OCR, the resultant damage to graft articular cartilage was affected by the delivered energy. (1) Delivered energy was decreased when a cushion was inserted between the drop mass and OCS, and total crack length of the cartilage surface was strongly correlated with delivered energy. (2) Higher tightness of graft-host fit resulted in higher cumulative energy delivered to the graft during insertion, as well as more cartilage damage. (3) With same tightness of fit, the OCG with modified geometry led to less energy delivery to the graft during insertion, with less resultant damage to cartilage. The experimental approach may be applied to a variety of impact insertion scenarios. The use of a cushion altered impact mechanics, mimicking certain aspects of graft insertion, and may be relevant to injury scenarios. Understanding the relationships between graft-host tightness of fit, graft geometry, and osteochondral graft insertion biomechanics may facilitate design for improved surgical instrument