Question

A (Co-Cr-Mo) hip implant was inserted into a patient’s femur. The acetabular cup was made of surgical stainless steel. After one year of operation, the patient was complaining about severe pain. He was put on painkillers because the situation was getting worse with time. X-ray radiography revealed wear debris as well as a large gap at the interface between the implant and bone. The orthopaedic surgeon decided to re-operate to retrieve the old implant and replacing it with a new one. He found massive corrosion in the retrieved implant.

i) What would be the causes of that corrosion, wear and the occurrence of the interfacial gap between the implant and bone?

ii) Describe the factors that should be taken into consideration in the design to minimize these failures.

Answer 1

i). acetabular debonding, a rare and poorly understood failure mode. In debonding, bone ingrows into the implant initially, but late loosening occurs when the plasma porous coat on the outer face of the acetabular cup becomes detached from the bulk implant and remains fixed to the pelvic bone. Acetabular component position seems to have some influence;...

identified postoperative time, AIA, and implant type as risk factors for debonding...

debonding of the porous coat occurs due to accumulation of microfractures at the coating-substrate interface.

Failures may relate to release of minute metallic particles or metal ions from wear of the implants, causing pain and disability severe enough to require revision surgery in 1–3% of patients. Design deficits of some prothesis models, especially with heat-treated alloys and a lack of special surgical experience accounting for most of the failures....

The cause of these failures remain controversial, and may include both design factors, technique factors, and factors related to patient immune responses (allergy type reactions).

Patients who have allergic reactions to cheap jewelry are more likely to have reactions to orthopedic implants. There is increasing awareness of the phenomenon of metal sensitivity and many surgeons now take this into account when planning which implant is optimal for each patient...

ii).

Initial hip designs were made of a one-piece femoral component and a one-piece acetabular component. Current designs have a femoral stem and separate head piece. Using an independent head allows the surgeon to adjust leg length (some heads seat more or less onto the stem) and to select from various materials from which the head is formed. A modern acetabulum component is also made up of two parts: a metal shell with a coating for bone attachment and a separate liner. First the shell is placed. Its position can be adjusted, unlike the original cemented cup design which are fixed in place once the cement sets. When proper positioning of the metal shell is obtained, the surgeon may select a liner made from various materials.

To combat loosening caused by polyethylene wear debris, hip manufacturers developed improved and novel materials for the acetabular liners. Ceramic heads mated with regular polyethylene liners or a ceramic liner were the first significant alternative. Metal liners to mate with a metal head were also developed

When currently available implants are used, cemented stems tend to have a better longevity than uncemented stems. No significant difference is observed in the clinical performance of the various methods of surface treatment of uncemented devices. Uncemented stems are selected for patients with good quality bone that can resist the forces needed to drive the stem in tightly. Cemented devices are typically selected for patients with poor quality bone who are at risk of fracture during stem insertion. Cemented stems are less expensive due to lower manufacturing cost, but require good surgical technique to place them correctly. Uncemented stems can cause pain with activity in up to 20% of patients during the first year after placement as the bone adapts to the device. This is rarely seen with cemented stems.

One of the most successful techniques in restoration of degenerated joint functions is total hip replacement. This surgical approach includes removal of diseased cartilage and bone parts, and replacement by the corresponding joint prostheses. By using metal alloys, high quality plastic and polymer materials, orthopedic surgeons can reconstruct hip fractures, or replace a painful, dysfunctional joint with highly functional, long-lasting prostheses, enabling hundreds of thousands of people to live a more fulfilling and active life. However, most of these implants only last for 10-15 years, and one of the most common problems for both patients and doctors is implant failure. Observed in long-term, implant loosening is the main cause of failure. Occasionally, dislocation or bending of implants may occur. Fatigue fracture and wear were identified as the main problems related to loosening of implants, stress shielding, and final implant failure. There are several factors which contribute to implant integrity, including material and design, implant positioning, cementing technique and patient characteristics. To improve integrity and life of hip implant by using finite element analysis, two major factor should be considered, design and biomaterial. By using this method there are various parameters that should be define, including complex geometry of the bone and an implant, biomaterial properties, and specifics boundary conditions, i.e. contact between the hip prosthesis and the bone depending on fixation method. In orthopedic biomechanics finite element method was first used for the purpose of determining the stresses in human bones. Since then, this method has been more and more frequently applied in determining of stress state in bones and prostheses, as well as fracture fixation devices, including hip implants. This paper presents a review study of factors influencing design process and structural integrity of the hip implant.