Diagnosis of spine pseudoarthrosis based on the biomechanical properties of bone
Authors: Hipp John A , Mikhael Mark M. , Reitman Charles A , Buser Zorica , Patel Vikas V. , Chaput Christopher D. , Ghiselli Gary , DeVine John , Berven Sigurd , Nunley Pierce , Grieco Trevor F 
Published: January 2024

Background

Cervical spine fusion, commonly performed with generally favorable outcomes, may result in postsurgical symptoms requiring further investigation and treatment. Anterior cervical discectomy and fusion (ACDF) aims to decompress neural structures, stabilize motion segments, eliminate intervertebral motion, and promote bridging bone formation. Failure to form bridging bone may result in persistent symptoms or symptomatic pseudoarthrosis. Traditional diagnosis involves computerized tomography to detect bridging bone and/or flexion-extension radiographs to assess whether segmental motion is above specific thresholds. This paper proposes a new biomechanically based diagnostic approach to address limitations in traditional diagnostic methods. The scientific basis of this approach is that bridging bone cannot occur if the strain is greater than the failure strain of the bone.

Methods

Fully automated methods were used to measure disc space strains. Errors in strain measurements were assessed from simulated radiographs. Measurement error combined with the reported failure strain of trabecular bone led to a proposed strain threshold for pseudoarthrosis diagnosis post-ACDF surgery. A reanalysis of previously reported flexion-extension radiographs for asymptomatic volunteers was used to assess whether flexion-extension radiographs, in the absence of fusion surgery, can be expected to provide sufficient stress on motion segments to allow for reliable strain-based fusion assessment. The sensitivity and specificity of strain- and rotation-based pseudoarthrosis diagnosis were assessed by reanalysis of previously reported post-ACDF flexion-extension radiographs, where intraoperative fusion assessments were also available. Finally, changes in strain over time were explored through the use of 9,869 flexion-extension radiographs obtained 6 weeks to 84 months post-ACDF surgery from 1,369 patients.

Results

The estimated error in measuring disc space strain from radiographs was approximately 3%, and the reported failure strain of bridging bone was less than 2.5%. On that basis, a 5% strain threshold is proposed for pseudoarthrosis diagnosis. Good-quality flexion-extension radiographs can be expected to stress the spine sufficiently to facilitate strain-based diagnosis of pseudoarthrosis. Reanalysis of a study in which intraoperative fusion assessments were available revealed 67% sensitivity and 82% specificity for strain-based diagnosis of pseudoarthrosis, which is comparable to rotation-based diagnosis. Analysis of post-ACDF flexion-extension radiographs revealed rapid strain reduction for up to 24 months, followed by a slower decrease for up to 84 months. When rotation is less than 2 degrees, the strain-based diagnosis differs from the rotation-based diagnosis in approximately 14% of the cases.

Discussion

Steps for standardizing strain-based diagnosis of pseudoarthrosis are proposed based on the failure strain of bone, measurement error, and retrospective data. These steps include obtaining high-quality flexion-extension studies, the application of proposed diagnostic thresholds, and the use of image stabilization for conclusive diagnosis, especially when motion is near thresholds. The necessity for an accurate diagnosis with minimal radiation exposure underscores the need for further optimization and standardization in diagnosing pseudoarthrosis following ACDF surgery.