Multi-Centre Study in patients undergoing Total Hip Arthroplasty with the Smith+Nephew CATALYSTEM™ Primary Hip System. The primary objective is to establish the mean inferior stem migration (subsidence) of the CATALYSTEM™ stem using CT-RSA over the first two postoperative years.

This study is designed as a prospective, multi-center, non-controlled study of subjects implanted with the CATALYSTEM™ Primary Hip System for primary Total Hip Arthroplasty (THA). Subjects will be enrolled and followed for 2 years post-operatively.

Radiostereometric analysis (RSA) is a tool to measure implant migration, originally developed in the 1970s. It is now well-established that early migration (micromotion) of implants with respect to bone is a predictor for later aseptic loosening. Traditional RSA used dual x-ray to estimate implant migration and has been standardized. CT-based methods (CT-RSA), which do not require metal reference beads have been developed since, and have been validated with equivalent results. International guidelines have since been updated to reflect this move to CT-RSA.

The primary objective is to establish the mean inferior stem migration (subsidence) of the CATALYSTEM™ stem using CT-RSA over the first two postoperative years.

Inclusion Criteria:

1. Patient requires primary unilateral THA for a severely painful and/or disabled joint from osteoarthritis, traumatic arthritis, rheumatoid arthritis, avascular necrosis, or congenital dysplasia.
2. Patients is at least 21 years old at the time of informed consent.
3. Patient is able to read and comprehend the informed patient consent document and willing and able to provide informed patient consent for participation in the study and have authorized the transfer of his/her information for study purposes.
4. Patient is able and willing to return for follow-up as specified by the study protocol.
5. Patient is able and willing to complete the required questionnaires and patient-completed portions of the study protocol.

Exclusion Criteria:

1. Patient requires implants other than the planned components or would not be a candidate for allowable components to be used (see 4) in the judgement of the surgeon.
2. Patient is planned to have simultaneous bilateral hip arthroplasty
3. Patient had contralateral hip arthroplasty less than 6 months prior to the day of surgery.
4. Patient is expected to have contralateral hip arthroplasty in the following 6 months at the time of consent into this study.
5. Patient has active local or systemic infection.
6. Patient has amputations in either leg that would impact rehabilitation following surgery.
7. Patient has loss of musculature, neuromuscular compromise or vascular compromise that would impact rehabilitation following surgery.
8. Patient has poor bone quality, such as osteoporosis, where in the surgeon's opinion, there could be considerable migration of the prosthesis, or require additional fixation, or a significant chance of periprosthetic fracture, or the lack of adequate bone to support the implant(s).
9. Patient has Charcot's or Paget's disease.
10. Patient is pregnant or lactating.
11. Patient is bedridden.
12. Patient has participated in a clinical investigation with an investigational product (drug or device) in the last 3 months.
13. Patient is currently involved in any personal injury litigation, medical-legal or worker's compensation claims.
14. Patients is, in the opinion of the surgeon, a drug or alcohol abuser or has psychological disorder(s) that could affect their ability to complete patient-reported questionnaires or be compliant with follow-up requirements.
15. Patient is diagnosed and taking prescription medications to treat a muscular disorder that limits mobility due to severe stiffness and pain such as fibromyalgia or polymyalgia.