Offset

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