Two cases of assymptomatic hip dislocation discovered incidentally are presented.

Case 1- A 63-year old lady had an uncemented primary total hip replacement of the left hip for painful osteoarthritis in July 1993. She made an uneventful recovery post-operatively. This lady had no neurological abnormality and was mobilising independently. In April 2001,8 years later she was admitted as an emergency for suspected diverticulitis of the colon. Plain radiographs performed showed dislocated hip prosthesis.

Case 2- This 75-year old lady, an active farmer, had right hip arthroplasty in July 1990 for painful osteoarthritis and made an uneventful recovery subsequently. She also did not have any neurological abnormality and was mobilising independently too. Dislocated prosthesis was discovered radiologically in December 2001 during a pre-operative work-up for the left hip (the other hip) arthroplasty.

Discussion: Late dislocation is more common than was thought previously. Several separate processes, some distinct from those associated with early dislocation, can lead to late dislocation. It can occur in association with a long-standing problem with the prosthesis that manifests late (such as malposition of the implant or recurrent subluxation), it can occur in association with a new problem (such as neurological abnormality, trauma or polyethylene wear), or it can occur in association with combination of these factors.

Both these patients were mobilising independently and did not suffer from any neurological abnormality. Both these patients had asked to be discharged after an initial 2-year follow-up. They had not experienced any problem with the hip replacement. These dislocated prosthesis were discovered incidentally. Revision arthroplasty was carried out successfully in both these patients

These cases emphasise the need for long-term clinical and radiological follow-up in hip arthroplasty patients as hip dislocations can be assymptomatic and not detected by clinical examination. Radiological review alongside evaluation using scoring systems is recommended.