Introduction

Basic Science
Anatomy
Biomechanics

Mechanism of Injury

Prevention

Gender Issues in ACL Injuries

Main Table of Contents

There are a few clinically relevant facts from basic science about the anterior cruciate that you should know.

The anterior cruciate ligament and posterior cruciate ligament are closely intertwined, and called the "central pivot". An injury to either ligament disrupts the function of the joint, and may lead to late degenerative arthritis. This relationship between the anterior cruciate ligament and the posterior cruciate ligament may be seen in this video

Tensile Strength

The ultimate failure load has been reported by Noyes to be 1750 Newtons. He noted the young specimens to be stronger than the older ones.

Visco - Elasticity

The speed of the force effects the type of tear. The slow twisting ski injury will elongate the ligament up to 30% before rupture. There may be partial rupture of antero medial bundle leaving the postero lateral intact .

If the patient is involved in low demand activities this partial injury may be adequate for stability. We examined 20 recreational athletes who were diagnosed as having a partial tear.

• This may have been a partial tear or partial healing of a complete tear. 2 years after their injury only one patient had to have a reconstruction.
• In the high demand athlete the bundle could be augmented with the semi-t . This technique preserves the proprioception.
• The athlete in football or basketball who stops and pivots quickly will usually have a mid substance tear. This will require a reconstruction as there is no potential for healing.

The Isometric Points

The isometric points of insertion of the tibia and femur must be known to drill the proper tunnels. Roger Larson has computer mapped these points.

• The femoral tunnel is 7 mm in from the drop off at the 11 or 1 o'clock position.
• If a 10 mm tunnel is drilled at this point then a 1-2 mm posterior wall will be left.
• The tibial site is 7 mm from the leading edge of the PCL at 70º of knee flexion.
• The ligament of Humphrey and fat on the PCL must be taken into account in this measurement.

This drawing outlines the tibial attachment of the ACL. The area of the tibial tunnel that is outlined is the most isometric point of the ligament.

The average length of the intra-articular ACL is 31 mm. This is important when preparing the length of the semi-t and patellar tendon graft. The total length of the tibial tunnel, the intra-articular length, and the femoral tunnel must be compared to the graft length, to avoid mismatch in the endoscopic procedure. This calculation avoids the protruding graft on the tibial side. The overall length of the graft should be 9-10 cm in total.