Simulation and Visualization Research Group
Computer-assisted compression hip screw placement
Introduction
The compression hip screw is the most common implant, within the UK,
for fixation of intertrochanteric fractures of the femoral neck.
Accurate guide wire placement is a prerequisite of accurate implant
placement, yet can be difficult to achieve for the less experienced
surgeon. The Computer Assisted Orthopaedic Surgical System (CAOSS)
described herein assists the surgeon during this technically demanding
phase of the procedure
The objective of this new approach is to aid the orthopaedic surgeon
in planning and realising the guide wire’s trajectory in relation to
the patient’s anatomical features.
CAOSS architecture
CAOSS consists of 3 modules: an enhanced fluoroscopic image intensifier
imaging system; an implant specific placement planning module; and a
passive manipulator arm.
Registration
Two near orthogonal x-ray images containing proximal femur and registration
phantom are distortion corrected and processed.
Distorted image
Corrected image
Features of interest are extracted and the image registered
in 3D space, through evaluation of the phantom's projection
Images of phantom overlaid onto the proximal femur
Phantom supported by End Effector
Planning
A computer-based model of the anatomical region is developed
and the position of the implant planned
Lateral image
Ap image
Versatility of CAOSS is increased with the provision of
an adjustment to the planned trajectory to the surgeon's satisfaction
Implementation
The surgeon implements the plan by moving the passive
manipulator arm, whilst receiving visual positional cues from the computer.
End Effector, supporting Cannula, whilst attached to Passive Arm
Once positioned the passive manipulator arm is locked
in place to support the guide wire insertion process.
Computer guidance.
The target is white and the ends of the cannula are red
and green
Results
Enlarged cross section through head, showing the placement
error of the guide wire, including any drill drift. Measurements are
obtained from a Co-ordinate Measurement Machine (CMM).
Cross section through head, with overlaid guide wire position.
Discussion
CAOSS has proved accurate and reliable in laboratory trials
on both anatomical and plastic bone specimens. In addition CAOSS requires
minimal x-ray radiation exposure. CAOSS is currently installed
in theatre for prospective randomised clinical trials, for which it
has full MDA, clinical and ethical approval. Distinguishing features
of CAOSS are:
CAOSS not only has the potential for improving patient
outcome but also for improving implant design. The capability
for accurate implant placement and recording of its final position relative
to the patient’s anatomy is invaluable to future implant development.
Although initially targeted at the femoral neck fixation, CAOSS is generic
technology that is suited to at least 12 other common orthopaedic procedures
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