Abstract
Aim: To determine the intra operative biomechanical properties of a semitendinosus graft used in ACL reconstruction.
Introduction: ACL reconstruction has become a commonly performed operation with 1,139 of these procedures being performed in South Australia in 1997 (SA Health Commission)
The majority of the scientific literature is based on data obtained from elderly cadaveric material. Little is known about the biomechanical properties of the soft tissue grafts currently used prior to implantation. The correct preconditioning and intraoperative tensioning of the soft tissue grafts has also not been investigated.
The initial graft biomechanical properties are important. Inadequate tension will lead to continuing instability whilst excessive tension may cause accelerated joint arthrosis. The tension in the graft may decrease by 30% if it has not been cyclically pretensioned.
Methods: A machine has been designed that will allow the intraoperative biomechanical testing of soft tissue grafts immediately prior to their implantation into the patient during ACL reconstruction. Data will be available on creep, stress relaxation, and tensile testing.
This device will also allow the accurate preconditioning of the graft, providing objective data that can then be compared to the subsequent clinical progress of the patient.
All testing will be accomplished during the time it takes to prepare the tunnels for insertion of the graft, and as such will not prolong unnecessarily the operative time.
Procedure: Once the graft has been prepared prior to fixation, it will be placed between two clamps. One is fixed to a load cell whilst the other is coupled to a linear actuator. The linear actuator will be driven by a computer controlled stepper motor under close-loop control. Custom software will cyclically load the autograft between two definable load points. A linear variable differential transformer (LVDT) will be used to monitor displacement of the autograft and load will be monitored with a load cell of capacity 125Kg.
This set-up will be immersed in a saline water bath maintained at body temperature during testing. The load cell will be hermetically sealed, with clamps and water bath being autoclavable. With the facilities for draping, the test area will remain sterile. The auto graft clamps will be designed to allow fixation of various graft materials (eg semitendinosus, gracilis, bone-patella tendon-bone) and adjustable for graft lengths. The water bath will house a thermocouple, heating mat and controller to maintain the saline temperature to within 1°C.
The testing system will be mounted on a stainless steel trolley for mobility in the operating room with an underlying shelf to house the associated electronics and a retractable side draw for storage of the laptop computer.
The autograft will be preconditioned between two known loads for 20 cycles recording load and displacement simultaneously on a laptop computer. Once preconditioned, the autograft will then be used for the ACL reconstruction in the standard way.
Summary: Objective data on preconditioning of ACL grafts, has never before been available intra-operatively. We outline the experimental set-up which has been designed and is undergoing testing prior to its use in a prospective study.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
