Introduction: Accurate recovery of humeral head geometry in shoulder arthroplasty is an important requirement for a good functional outcome. Despite this, spherical prosthetic components are implanted when the total proximal humerus is described as ovoid1. However, 60 to 80 % of the head is spherical1. If, in the normal glenohumeral joint, only the spherical portion is in contact with the glenoid then recovery of normal mechanics is likely with a spherical prosthetic component.
Contact patterns have been examined ex vivo2 under static conditions but do not reflect the likely in vivo contact pattern under dynamic loading and have not been correlated to changes in sphericity of the articular surface. A recent study of the distal femur found that thickness of normal articular cartilage is positively correlated with loading3 and, thus, contact.
The objective of this study was to determine the feasibility of using a surface laser scanner to determine cartilage thickness and, therefore, likely contact area and to correlate changes in thickness to changes in sphericity of the articular surface.
Methods: A cadaveric arm without bony deformity or evidence of rotator cuff disease was dissected free of soft tissue and mounted on a rigid block within the frame of a surface laser scanner (Kestrel3D Ltd., UK). The articular surface of the humerus was scanned at a resolution of 200 μm. The cartilage was then dissolved away and the humerus re-scanned. The x,y,z coordinate data of the re-scanned bone were used to match the sub-chondral bone with the cartilage from the previous scan using Pointstream™ software (Kestrel3D Ltd., UK).
The cloud point data for the cartilage and bony surfaces were exported into modelling software (McNeal and Assoc., Seattle, WA) and the surface area of the head divided into ten equal sections. For each slice of both the cartilage and bony surface, the radius of curvature was calculated using a least square fit optimisation technique4. The differences in radius of curvature between the cartilage surface and subchondral bone surface were used to calculate the cartilage thickness for each slice. The standard deviation from the radius of curvature was used to calculate the degree of deviation from sphericity.
Results: For the first 60 % of the surface area, the deviation from sphericity was 0.5% of the radius with a cartilage thickness of 0.74 mm. The deviation from sphericity and cartilage thickness for 100% of the articular surface was >
1% and 0.63 mm, respectively.
Conclusions: The experiment proved that the surface laser scanner can be used to elucidate the relationship between contact patterns and articular curvature of the proximal humerus. The changes in sphericity concur with results from previous studies1. Assuming cartilage thickness correlates to contact patterns at the normal glenohumeral joint, the change in cartilage thickness suggests that contact may occur only at the spherical portion of the head. Knowledge of this relationship may aid in future prosthetic design considerations or in modification of the osteotomy technique. To further support these findings, a 50μm laser scanner is being developed and will be used on a larger sample size.
