DIRECT MEASUREMENT OF HOOP STRAINS IN THE INTACT AND TORN HUMAN MEDIAL MENISCUS

Objective. To measure the circumferential or hoop strains generated in the medial meniscus during loading of the knee joint and to examine t he effect of longitudinal and radial tears in the meniscus on these st rain values. Design. An in vitro investigation measuring the circumfer ential strains in the medial menisci of cadaveric human knees as they were loaded in a materials testing machine. Background. The menisci tr ansmit approximately 50% of the load through the knee, the rest being transmitted by direct contact of the articular cartilage. Damage to th e menisci will alter the pattern of load transmission as will meniscec tomy. This study examined the changes in the mechanics of the meniscus in situ as a result of simulated tears to assess the effect of its lo ad carrying capacity and the implications of surgery to remove part or all of a damaged meniscus. Methods. Nineteen human cadaveric knees we re tested. Windows were made in the joint capsule and strain gauges in serted into the anterior, middle and posterior sections of the medial meniscus. The knees were then loaded to three times body weight at spe eds of 50 and 500 mm/min, with the knee joint at 0 degrees and 30 degr ees of flexion. The tests were repeated following the creation of a lo ngitudinal or a radial tear in the meniscus. Results. The intact menis ci showed significantly less strain in the posterior section compared to the anterior and middle sections (P<0.003, with strains of 1.54%, 2 .86% and 2.65% respectively). With a longitudinal tear this pattern ch anged with strains decreasing anteriorly and increasing posteriorly. T here were also significant differences at different angles of knee joi nt flexion not seen in the intact meniscus. 50% radial tears reduced t he strains anteriorly whilst a complete radial tear completely defunct ioned the meniscus. Conclusions. This study has shown that there are s ignificantly different hoop strains produced in different sections of the medial meniscus under load and the patterns of strain distribution are disturbed by meniscal tears. Relevance These results provide impo rtant data for mathematical models which must include nonuniform behav iour. They also have implications for the surgical management of torn menisci. Undamaged portions should be preserved and the integrity of t he circumferential fibres maintained to ensure the menisci retain a lo ad bearing capability. Copyright (C) 1996 Elsevier Science Ltd.