Over the last decade stemless shoulder arthroplasty has become increasingly popular. However, stability of metaphyseal loading humeral components remains a concern. This study aimed to assess the stability of the Affinis stemless humeral component using Radiostereometric analysis (RSA).

Patients underwent total shoulder arthroplasty via a standardised technique with a press-fit stemless humeral component and a cemented pegged glenoid. Tantalum beads were inserted into the humerus at the time of operation. RSA of the relaxed shoulder was completed at weeks 1, 6, 13, 26, 52 and 104 post-operatively. Stressed RSA with 12 newtons of abduction force was completed from week 13 onwards. ABRSA 5.0 software (Downing Imaging Limited, Aberdeen) was used to calculate humeral component migration and induced movement.

15 patients were recruited. Precision was: 0.041, 0.034, 0.086 and 0.101 mm for Superior, Medial, Posterior and Total Point Motion (TPM) respectively. The mean TPM over 2 years was 0.24 (0.30) mm, (Mean (Standard deviation)). The mean rate of migration per 3 month time period decreased from 0.45 (0.31) to 0.02 (0.01) mm over 2 years. Mean inducible movement TPM peaked at 26 weeks at 0.1 (0.08) mm, which reduced to 0.07 (0.06) mm by 104 weeks when only 3 patients had measurable inducible motion. There was no clear trend in direction of induced movement. There were no adverse events or revisions required.

We conclude migration of the humeral component was low with little inducible movement in the majority of patients implying initial and 2 year stability of the stemless humeral component.

Aims Intro: Radiostereometry (RSA) is a powerful tool for assessment of prosthesis migration in total hip replacement. Typically marker beads are inserted into the bone and into turrets inserted into the prosthesis. Where the prosthesis can not be beaded valuable information may be lost. We present two methods that determine the three-dimensional prosthesis position using geometric models. Method: The þrst method approximates the stem of the prosthesis to an axially symmetric cylinder and spherical head. The method is fast but may lead to errors due to the cylinder approximation. The second uses accurate three-dimensional surface models of each implant model. Clinical precision was determined from repeat same day RSA examinations. Results: Given as 99% precision limit rotation ranged from 0.4 degrees (sagittal) to 2.5 degrees (axial) for the cylinder method (n = 38), compared to 0.2 degrees (sagittal) to 0.6 degrees (axial) for the 3d model (n = 6). Translations were similar ranging from 0.2mm axial to 0.7mm sagittal. Conclusion: Model based RSA can give useful results where beads can not be implanted into the prosthesis. Although the primary method gives good results these are less precise than that for beaded RSA. Simulations infer a potential accuracy for the method of a few microns, however this is currently limited by the closeness of correspondence of an individual stem to its model. The method here is applicable to any implant type. Accurate 3D models are required, however for new designs these should be available.

Aims: To study prospectively the characteristic migration of four different cemented femoral stems using Radiostereometry.

Methods: 96 patients undergoing cemented femoral hip replacement for osteoarthritis were randomised to receive an Exeter (Howmedica Stryker), Ultima Tapered Polished Stem [TPS], Ultima straight stem [USS] (Johnson and Johnson) or Charnley Elite (Depuy/Johnson and Johnson) stem. RSA migration measurements were performed at postoperatively and at 6,12,18 and 24 months post surgery using the UMRSA system enhanced with locally developed software.

Results: Two significantly distinct patterns of distal migration in the y direction (subsidence) were seen compatible with previous studies. Tapered stems subsided rapidly by six months Exeter 0.85mm (+/−0.15) UTPS 0.85mm (+/−0.15) stabilising by 2 years Exeter 1.3mm(+/−0.35), UTPS 1.2mm(+/−0.25). The USS and Elite designs showed lower migrations of 0.1mm (+/−0.1) and 0.15 (+/−0.15) mm at six and 0.25mm(+/−0.15) and 0.35mm (+/−0.15) 24 months. For migrations and rotations in other directions differences were seen between prostheses with the same design concept. Whilst both the UTPS and Exeter exhibited posterior migration of the head, by 24 months this was expressed as a rotation about the transverse axis for the TPS and the axial axis for the Exeter. The Elite and USS differed in rates of posterior migration and varus tilt.

Conclusions: Patterns of migration for stems with the same design concept may vary due to subtle differences in prosthesis shape. These variations produce a distinct migration “signature” for each stem.