The normal and ACL deficient knee : an in vivo three dimensional kinematic and electromyographic analysis

Abstract

The overall aim of this thesis was to describe the in-vivo 3D kinematics of the normal and ACL deficient knee. Baseline data from normal controls were compared against the pathological knee to assess whether generic functional knee braces reduced abnormal displacements. Moreover, 3D bony contact movements were derived from MRI data.

Six patients with ACL rupture and 8 normal controls participated in the invasive pin experiments. During dynamic activity, 3D motion analysis was used to record knee kinematics using markers affixed to 3.2 mm Hoffman bone pins inserted into the tibia and femur. Simultaneous neuromuscular activity was measured for the patient group. Ground reaction force (GRF) data were control for to ensure consistent jumps. For the MRI study, recordings were taken of the right knee of sixteen normal subjects.

For the pin studies, all subjects moved their knees freely. None experienced significant discomfort. For the patient group, intra-subject GRF data was generally consistent between bracing conditions. Therefore, landings were considered similar. Intrasubject kinematics was repeatable but no consistent reductions in anterior translations were evident. Anterior displacements when unsupported were on average 4.4 mm (range 2.2 to 8.8 mm) and 3.6 mm (range 2.4 to 5.7 mm) with the knee braced. Neuromuscular changes were observed as a function of bracing. Semitendinosus activity significantly decreased 17% prior to footstrike (P<0.05). Bicep femoris significantly decreased 44% whereas rectus femoris activity significantly increased 21% following footstrike (P<0.05). These findings suggest joint stability may result from proprioceptive feedback rather than the mechanical stabilising effect of the brace. Therefore, knee bracing combined with proprioceptive and muscular coordination training to increase joint stability is important.

When normal controls were considered in combination with our brace study, peak vertical force values did not exceed 2 times bodyweight whereas the ACL deficient group experienced up to 3 times bodyweight. Displacement magnitudes observed for the non-braced and braced ACL deficient subjects were similar to normal controls. Mean anterior tibial displacements for normal controls were 3.7 mm (range 1.2 to 7.3 mm) and 6.1 nun (range 1.4 to 10.3 nun) for the jump and cut respectively. The level of activity may not have been adequate to provoke the large displacements in order to identify whether knee braces reduced pathological translations to within normal. For the MRI study, movement of the contact points between the medial and lateral condyles was posterior 12.5 mm vs. 13.8 mm, laterally 0.6 mm vs. 1.4 nun and inferiorly 6.2 mm vs. 9.7 mm when flexing to 30'. From 300 to 600, the contact points moved anteriorly 1.0 mm vs. 6.4 mm, continued laterally 2.5 mm vs. 2.4 mm, and progressed superiorly 2.4 mm vs. 4.7 nun. Further research is required to verify these findings during weight-bearing conditions.

Problems were associated with the femoral pin. During mechanical testing, loads of 150 N and 100 N applied at 15 mm and 20 mm respectively produced deflections larger than 0.4mm. A resonance frequency of 90 Hz was observed. By improving insertion and pin design, this technique may become more reliable.