Effect of intra-articular corticosteroids on ligament properties: a biomechanical and histological study in rhesus knees

Abstract

The effect of intra-articular corticosteroid injections (methylprednisolone acetate) on the mechanical properties of anterior cruciate bone-ligament-bone units was determined. Fifty-one wild rhesus animals provided 88 knee ligaments units that were loaded in tension to failure under fast strain-rate conditions. Alterations in ligament strength and load-elongation properties were found to depend upon corticosteroid dosage and time after injection. Fifteen weeks after the higher drug dosage (methylprednisolone acetate, 6 mg/kg; three injections, spaced weekly) significant decreases occurred in the maximum failure load (20%), energy absorption prior to failure (11%) and linear stiffness (11%) of the ligament unit. In contrast, only minimal non-significant alterations in ligament strength properties occurred at the higher drug usage after 6 weeks in a second group of animals. this strong time-dependence may partially explain the negative findings reported in short term studies. The higher drug dosage used is approximately 10 times an equivalent human dose on a body mass basis and indicates the ability of this class of drugs to significantly alter the mechanical properties of a ligament unity. A third group of animals received a total of two intra-articular coticosteroid injections (single injections spaced two weeks apart) at a dosage (0.6 mg/kg) equivalent to that commonly used clinically in humans. Statistically significant decreases occurred in maximum failure load (9%) and energy absorption (8%); however, this change is believed of little importance to the projected functional capacity of the ligament unit. No detectable systemic effect of the intra-articular corticosteroid injections was found on the mechanical properties of the ligament unit in the extremity opposite to that which received the drug injection. Histological examination after failure showed normal cellularity and staining characteristics of the ligament, fibrocartilage junction and underlying bone. No bone resorptive changes were observed beneath the ligament insertion site. Failure by a ligamentous mode remained the most common mechanism of specimen failure. Scanning Electron Microscopy showed failure of collagen fibers at multiple levels throughout the ligament indicative of a serial pull-apart failure process. A rich supply of vessels to the anterior cruciate ligament was demonstrated and failure of the vessels occured by a necking down process. Insofar as the results apply to humans, they infer the potential for high and frequent doses of intra-articular slightly soluble corticosteroids to produce alterations in ligament strength and function. A single intra-articular injection or one repeated at intervals of several months probably carries little risk to ligament properties.