Influence of new macromolecular lubricants

Quantification of wear morphologies of novel materials used in Total Hip  Arthroplasties: Influence of new macromolecular lubricants and different surface geometry,Presented by Dr. Somayeh Akbari & mrs. Maryam Iraji(1 st of March, 2021)

Webinar 1 March 2021

In order to approve a hip implant with new material for utilization in human body, scientists need to do a 4-stage process:

Stage 1: characterization of the new material and studying the mechanical and microstructural properties

Stage 2: in vitro testing and studying the possible toxicity of the material particles to living tissue and blood cells.

Stage 3: in vivo implantation in which a prototype implant functions in the body of lab rats and the study of possible damages is proceed.

Stage 4: after getting approval in all the previous 3 stages, the implant could go for human trials.

Before starting the design of an artificial hip joint we should be informed about the anatomy and biomechanics of natural hip joint.

A hip implant is a ball and socket synovial joint which have three degrees of freedom and can move in three planes. The head of the femur articulates in the acetabular cup in the pelvic and the contact surface is covered with articular cartilage and the joint is lubricated with synovial fluid which is produced by synovial membrane. This lubricant is produced even after the removal of natural joint and implanting the artificial one.

Any damage that violate the blood supply in the hip joint, causes inflammation and pain and leads patients to consider a hip surgery. Osteoarthritis is the most common joint disease in the world in which the gradual erosion of cartilage leads to complete exposure of bone; a degenerative pathology, involves irreversible damage and aging, misloading of joints, accidents, excessive physical activity or inactivity are contributing risk factors for this disease. There are different treatments for this problem. None operative and operative treatments. The none-operative treatment is very limited and is not functional in serious cases.

Operative treatment includes three different methods: hip resurfacing, hemiarthroplasty and total hip arthroplasty.

The reported benefits of hip resurfacing (HR) include reduced dislocation rate and increased function compared to total hip arthroplasty (THA). However, femoral neck fracture and stress shielding remain a concern with resurfacing hip prostheses. Stress shielding may lead to implant failure, including implant loosening and femoral neck fracture.

This surgical procedure replaces the head of a damaged femur with an implant designed to stabilize the femur and restore hip function. Unlike total hip replacement, in which both the ball of the femur and the hip socket are replaced, in this procedure, only the ball is replaced. HA has advantages of the shorter operation time, less blood loss, less technical demand, less economic burden, and a lower dislocation rate. the frequency of HA procedures increases with age, whose cutoff is at 76, due to, in part, the contradictions between the more extensive surgery of THA and the lower surgical tolerance of elderly patients. This method benefits from two different type of design, unipolar which is consist of a stem and a femoral head; bipolar which is a unipolar implant with a femoral head which is able to rotate in the acetabular cup. According to real clinical studies there is no significant difference between the function of these two designs.

Since the so called HA is not suitable for osteoarthritis patients, it has symptomatic acetabular protrusion or erosion, post-operative thigh and groin pain has been observed and it is not suitable for active patients; it is recommended to use the THA. In this method the acetabulum and femoral head is replaced with artificial joint. Materials in this design are either hard on hard (a metal or ceramic femoral head articulating against a metal or ceramic cup) or hard on soft (in which a metal or ceramic head articulate against a polymeric liner). Many factors influence the choice between these types of bearing, such as the implant cost, age and activity level of the patient, complications during surgery etc.

PTFE is one of the early polymers used in THA that has been rejected after observing allergic reactions to the wear debris and UHMWPE gained attentions because of its good tribological properties.

Metal on Metal (MOM) articulations were used for both total hip replacements and hip resurfacing (HR), which have the advantage of preserving the femoral head and neck, resulting in a less invasive operation and a lower dislocation rate. The issues of metal debris came to light, the MOM replacements were almost stopped completely.

Ceramics are known to be the best wear resistant materials to date and they have shown great function in clinical devices. However they have shortcomings like weak impact resistance and they fracture in case a crack occurs.

It is well known that wear tests that are close to the in vivo conditions have a long duration and elevate costs. The wear simulation is run for several million cycles, considering that one million cycles corresponds to one year in vivo. Wear is considered the most crucial issue in femoral head and the efforts are to characterize materials more resistant to wear. The pin/ball on disc is a conventional wear test that can be used for wear and friction measuring of materials. Since the material that is used for implants faces different type of loadings, hip and knee simulators can better replicate the contact pressure and motion profiles to be more like those encountered by artificial joints in vivo. However for the friction tests the conventional pin on disc simulators are more reliable.

The common problem for both mechanisms is to find a proper simulating environment. Influencing factors for this purpose are sliding velocity, contact pressure, hard particles (like cement, bone or wear debris) and lubricant. Lubricant is a very important factor for the in vitro simulation test, and it could affect the testing results in many ways. It has been reported that wear test in dry condition would produce a different wear mechanism. In general, there are three kinds of lubricants that have already been used for testing artificial joints: deionized water, saline solution and serum. However none of these can resemble the synovial fluid and in vivo condition perfectly and efforts are to produce a lubricant with similar properties.

Surface roughness is an important index for artificial joints since it has a significant influence on the tribological performance of bearing surfaces by trapping wear particles, reserving lubricant and enabling hydrodynamic pressure which all result in less wear rate.

Three-dimensional printing can be used for patient-specific therapy, as it allows for the fabrication of custom-made implants and medical devices. It is a suitable technique to create patient-specific anatomical models, customized molds, and surgical guides, as well as permanent implants. It allows better surgical planning, creates customized patient specific implants, shortens surgical time, and hospital stay.