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Pin loosening in a halo-vest orthosis: a biomechanical study
B C Fleming1, M H Krag, D R Huston
1McClure Musculoskeletal Research Center, Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, VT 05405, USA. fleming@salus.med.uvm.edu
Study Design:
The cranial pin force history of a halo-vest orthosis was measured using an instrumented halo in a clinical study with three patients. Pin force values at the time of halo-vest application and at subsequent clinical visits during the halo-vest wear period were compared.
Objectives:
To document the pin force reduction in the cranial pins of a halo-vest orthosis in vivo.
Summary Of Background Data:
The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. An important problem with halo-vest use is pin loosening. There have been no previous reports of pin force history in vivo.
Methods:
A custom-built strain-gauged, open-ring halo was used to measure the compressive force and superiorly-inferiorly directed shear forces produced at the tips of the two posterior pins. The instrumented halo was applied to three patients with cervical spine fractures. Pin force measurements were recorded at the time of halo application and at subsequent follow-up visits during the entire treatment period.
Results:
A mean compressive force of 343 +/- 64.6 N was produced at the pin tips during halo application with the patient in a supine position. On average, the compressive forces decreased by 83% (P = 0.002) during the typical halo-vest wear period. The compressive forces were substantially greater than the shear forces, which averaged only -11+/-30.2 N at the time of halo application and which did not change significantly with time.
Conclusions:
The study confirmed the hypothesized decrease in the compressive pin forces with time. All patients had developed at least some clinical symptoms of pin loosening at the time of halo-vest removal.