Which factors influence proximal femoral asymmetry?

Aims

In patients where the proximal femur shows gross deformity due to degenerative changes or fracture, the contralateral femur is often used to perform preoperative templating for hip arthroplasty. However, femurs may not be symmetrical: the aim of this study was to determine the degree of variation between hips in healthy individuals and to determine whether it is affected by demographic parameters.

Materials and Methods

CT-scan based modelling was used to examine the pelvis and bilateral femurs of 345 patients (211 males, 134 women; mean age 62 years (standard deviation (sd) 17), mean body mass index 27 kg/m2 (sd 5)) representing a range of ethnicities. The femoral neck-shaft angle (NSA), femoral offset (FO), femoral neck version (FNV), femoral length (FL), femoral canal flare index (fCFI), and femoral head radius (FHr) were then determined for each patient. All measurements were constructed using algorithm-calculated landmarks, resulting in reproducible and consistent constructs for each specimen. We then analyzed femoral symmetry based on absolute differences (AD) and percentage asymmetry (%AS) following a previously validated method.

Results

We found an asymmetry > 2% for NSA (mean AD 2.9°, mean %AS 2.3; p = 0.03), FO (AD 3.8 mm, %AS 9.1 ; p = 0.01), FNV (AD 5.1°, %AS 46.7 ; p = 0.001) and fCFI (AD 0.2 mm, %AS 5.4 ; p = 0.7). Significant relationships were found for AD regarding NSA and ethnicity (p = 0.037), FL and height (R2 = 0.22), and fCFI and gender (R2 = 0.34).

Conclusion

Our data confirm the presence of asymmetry of proximal femurs, which is mostly independent of demographic parameters. In cases where contralateral templating is used, such asymmetry may lead to inaccurate anatomical restoration of the hip if the templated sizes are routinely implanted. However, the clinical impact cannot be determined from our investigation. Cite this article: Bone Joint J 2018;100-B:839–44.

Spondylolysis originates in the ventral aspect of the pars interarticularis: a clinical and biomechanical study

Lumbar spondylolysis is a stress fracture of the pars interarticularis. We have evaluated the site of origin of the fracture clinically and biomechanically. Ten adolescents with incomplete stress fractures of the pars (four bilateral) were included in our study. There were seven boys and three girls aged between 11 and 17 years. The site of the fracture was confirmed by axial and sagittal reconstructed CT. The maximum principal tensile stresses and their locations in the L5 pars during lumbar movement were calculated using a three-dimensional finite-element model of the L3-S1 segment. In all ten patients the fracture line was seen only at the caudal-ventral aspect of the pars and did not spread completely to the craniodorsal aspect. According to the finite-element analysis, the higher stresses were found at the caudal-ventral aspect in all loading modes. In extension, the stress was twofold higher in the ventral than in the dorsal aspect. Our radiological and biomechanical results were in agreement with our clinical observations.

Minimally Invasive Decompression for Lumbar Spinal Canal Stenosis in Younger Age Patients Could Lead to Higher Stresses in the Remaining Neural Arch - A Finite Element Investigation

BACKGROUND AND PURPOSE

A young patient group with the symptoms of acquired spinal stenosis has been identified recently in the literature. The patients between 25-50 years of age were found to have signs of lumbar spinal stenosis because of degenerative spinal changes. Some of them were operated on using the same limited decompression approaches as the older patients. However, this group differs from the geriatric population due to the scarcity of remodeling degenerative signs at the spine. Therefore, the possible ligamentous laxity, facet joint degeneration or only the removal of some spinal structures could lead to the increased stresses in the remaining spinal arch and could have an unfavorable course of events after the procedure. A biomechanical study has been done using an experimentally validated finite element model (FEM) of the intact L3-S1 lumbar spine to elucidate the influence of the limited decompression on range of motion (ROM) and stress distribution on the neural arch in this patient group.

METHODS

We simulated unilateral laminotomy L4 and medial facetectomy L4-5, medial facetectomy L4-5 and lateral fenestration of L5 pars interarticularis, combined transarticular lateral and medial approach with partial facetectomy L4-5, "port-hole" decompression at the L4 level, and hemilaminectomy L4 with medial facetectomy L4-5. The ROM and maximum von Mises stresses were analyzed in flexion, extension, lateral bending, and axial rotation in response to a 10.6 Nm moment with 400 N axial compression. The data were compared with the intact spine and hemilaminectomy L4 with medial facetectomy L4-5 models.

RESULTS AND CONCLUSION

The investigation revealed almost the same ROM after simulation but a considerable increase in stresses at both the pars interarticularis and the inferior facet after limited decompressions, especially in extension and rotation to the contralateral side. Stresses at the contralateral L4 pedicle were highest after L4 hemilaminectomy and medial facetectomy L4-5. Due to the observed increases in stresses, the surgeon should be aware of the possibilities of stress-fractures in this patient group.

Role of mechanical factors in the evaluation of pedicle screw type spinal fixation devices

Prior to implantation, spinal implants are subjected to rigorous testing to ensure safety and efficacy. A full battery of tests for the devices may include many steps ranging from biocompatibility tests to in vivo animal studies. This paper describes some of the essential tests from a mechanical engineering perspective (e.g., motion, load sharing, bench type tests, and finite element model analyses). These protocols reflect the research experience of the past decade or so.