# The Effects of Lumbar Spinal Fusion on Spinopelvic Biomechanics Years After Primary Total Hip Arthroplasty: A Case Report

## TL;DR

A patient's hip stability declined years after a successful hip replacement following spinal fusion surgery, highlighting biomechanical changes.

## Contribution

This case report demonstrates how lumbar spinal fusion can lead to hip instability years after total hip arthroplasty.

## Key findings

- Lumbar fusion reduced pelvic mobility and increased risk of hip dislocation.
- Posterior hip dislocations occurred during forward flexion after spinal fusion.
- Strategies for managing hip instability in patients with spinal stiffness are discussed.

## Abstract

Spinopelvic biomechanics are critically important for hip stability following total hip arthroplasty (THA). Lumbar spinal fusion, particularly multilevel fusion, alters the dynamic relationship between the spine, pelvis, and hip, resulting in reduced pelvic mobility and increased risk of prosthetic instability. We present the case of an 85-year-old woman with a well-functioning right THA that remained stable for 11 years until she underwent L2-L5 laminectomy and posterior spinal fusion with L4-L5 transforaminal lumbar interbody fusion. Within the subsequent two years, she experienced two distinct episodes of posterior prosthetic hip dislocation, each occurring during forward flexion. This case highlights the biomechanical consequences of lumbar fusion, reviews the underlying mechanisms of altered spinopelvic mobility, and discusses strategies for preventing and addressing hip instability in patients with spinal stiffness or fusion.