Definition
The perpendicular distance from the centre of the femoral head to the long axis of the femur
Harris 1992
- aim for supra-physiologic offset
- avoid making offset less than original at all costs & makes longer if possible
Soft tissue balancing equals restoring femoral offset
Effect of short offset
1. Abductor lurch / Trendelenburg gait
2. Increased JRF / increased wear rates
3. Weak abduction increases O2 consumption
4. Impingement
5. Lax soft tiiues can cause dislocation
Decreased offset leaves the abductors lax
- doing so means they have to act with increased force
- increases the joint reaction forces across the hip.
- rsult of this is increased wear rates
Sakalkale et al Clin Orthop 2001
- 17 patients bilateral THR
- one STD, one high offset
- liner wear 0.21mm c.f. 0.01 mm /year
Increased offset
Advantage
- decreases JRF
- tightens lax abductors
Disadvantage
- theoretical increase in torque forces on stem and cement in flexion
- cadaver studies demonstrate increased offset doesn't increase torque forces on cement & bone
- torque increased in the stem but it is below the fatigue threshold modern stems
Factors affecting offset
- neck shaft angle
- head neck length
- anteversion
- femoral osteotomy level
- position of acetabulum
How to increase offset
1. Decreasing neck shaft angle
- more varus neck shaft angle
- increases torque on implant
2. Increasing Head / Neck length
- improves abductor tension
- worsens LLD
3. Medialising femoral neck whilst lengthening femoral neck
- technique in high offset stems
- maintains neck shaft angle
4. Advancing GT
- increases abductor offset
5. Acetabular component
Lateralised liners
- increase offset whilst preserving leg length
- can worsen body weight lever arm
- do so only when increasing femoral offset insufficient
Medialising centre of rotation
- decreases offset
Checking Soft Tissue Tension and Offset
1. Preoperative templating
- normal side
- aiming to reproduce normal biomechanics
A. Limb length
B. Acetabular component
C. Femoral component
2. Intraoperative measurement
Measurement jigs
- 2 fixed reference points
- limb in consistent position
- measure length and offset
- usually pin in supra-acetabular area
- second in GT
3. Intraoperative maneuvers
Shuck test
- distraction of hip joint with in line traction
Dropkick test
- hip extended, bend knee to 90o
- if too tight, RF is taut and passively extends the knee
Leg to leg comparison
- feel knees when legs is similar positions
- feel tension of abductors
Rotation
- external rotation in extension
- flexion