PROTHESES

Joint replacements are mechanical devices designed to replace, in whole or in part, a joint affected by highly debilitating degenerative diseases such as osteoarthritis, restoring functionality

There are prostheses for all joints: the most used are those for the hip and knee.

Less used are those for the shoulder, ankle, elbow, wrist and joints of the hand.

The materials used for the realization of these prosthetic components (biomaterials) are mainly metal alloys of titanium, chromium cobalt molybdenum, steel, bio ceramic and very high molecular weight polyethylene materials in clinical use for over 50 years, and which have demonstrated excellent biocompatibility and reliability.

Prosthetic components can be attached directly to the bone (biological anchorage) or by using special acrylic cements (cemented prosthesis).

The choice between these two options is made by the orthopedic specialist after the evaluation of different parameters such as the biological age of the patient, the degree of activity of the same, and the quality of the bone.

HIP PROTHESIS

Hip implants are an excellent solution in the surgical treatment of chronic degenerative bone and joint diseases such as osteoarthritis, arthritis, avascular necrosis of the femoral head or traumatic diseases (femoral neck fracture in the elderly).

Prosthetic hip replacement surgery, introduced in the second half of the last century, after an initial pioneering stage have become routine in most hospitals.

Experience gained in 50 years of clinical use have contributed to the current standards in terms of materials, instruments and surgical procedures making this type of intervention effective, reliable and durable.

Current statistics attest to 15 years, more or less, as the average life of a hip replacement. The duration of a  arthroplastic implant however depends on many factors, variables in the individual, such as the physical condition and body weight, activity level, metabolic factors. In addition, surgical technique and experience of the operator play a major role in the success of the system.

Hip prostheses are generally composed of four elements:

The femoral stem: implanted into the proximal part of the femur after resection of the femoral head and the preparation of the seat by means of special rasps.

The articular head: is inserted on a conical design on the neck of the stem.

The Cup, or acetabular cup: is implanted into the acetabular cavity of the pelvis after preparation of the bone seat by means of special reamers.

The joint insert: is inserted interlocking into the acetabular cup having a concave seat in which it is housed and articulates with the joint head.

An artificial hip, even though made up of high quality materials, is not as strong and durable as a natural healthy hip and there is no guarantee that it will last for the rest of the patient’s life. Articular components however, are subject to wear and will necessarily need replacement with new components.

In particular,  wear affects the acetabular cup insert, normally polyethylene: the movement of the femoral head in its interior, matched to the load, determines the wear that leads to having to “revise” the implant.

Aiming to prolong the useful life of the artificial joint as much as possible by postponing a possible revision surgery, in recent years  material innovations has been introduced to minimize or avoid the phenomenon of wear:

  • highly cross-linked polyethylene inserts (more wear-resistant) also added with anti-oxidant agents like vitamin E ;
  • articular combinations (head / insert) with very low wear rate, like ceramic-on-ceramic (head and insert bio ceramics).

 

KNEE PROTHESIS

Knee replacement products were introduced after the hip replacement prostheses.

Their systemic diffusion began in the late 70s of last century. Over the last 20 years clinical results have reported good or excellent outcomes (recovery of normal function with little or no pain relief) in 90-95% of cases, with survival of the implants at over 10 years.

These implants substantially, are to be considered resurfacing, as they replace the femur and tibia articular surfaces damaged by degenerative disease without being anchored within the bone as in the case of hip stems (knee prostheses anchored by intramedullary stems are used only in special cases or revision).

A total knee prosthesis is composed of 3 elements:

A femoral component, metal which is fixed to the distal part of the femoral condyle.

A tibial component, metal which is fixed to superior face of the tibial plate.

A polyethylene articular insert, that is fixed to the tibial plate and interposes between the tibial plate and the two metal components acting as a “meniscal”.

A fourth component can be added, the patella, which is fixed to articular face of the patellar surface.

 

Even an artificial knee joint, made up of high quality materials, is not as strong and durable as the natural healthy knee and there is no guarantee that it will last for the rest of the patient’s life. The articular components are subject to wear and it will be necessary to replace them with new components.

Also in this case, wear affects the polyethylene joint insert: the movement of the femoral component on the insert, coupled with the load, determines the wear that leads to having to “revise” the implant.

Contrary to what occurs in the hip replacement, for the knee component coupling with very low wear (ceramic-on-ceramic and metal-on-metal) is not contemplatable. To minimize polyethylene wear an intervention on the material itself has taken place, introducing the use of highly cross-linked polyethylene, and a high grade finish on the articular surfaces metal (Permedica applies innovative finishing technology MICROLOY®), obtaining an articular surface with roughness values extremely low so that they reduce friction and drastically reduce the levels of wear.

CONSERVATIVE PROSTHESIS

The increase in life expectancy, resulting in demand for good quality of the same, and improvements in techniques and materials used, have led a growing number of patients suffering from joint diseases to resort to joint replacement even in a relatively young age.

These patients, still very active and with a long life expectancy, are in need of “conservative” prosthetic solutions, that target bone stock preservation as much as possible in anticipation of one or more future revisions.

Given their characteristics, these prostheses also allow the use of “minimally invasive” implant techniques which provide surgical incisions of reduced dimensions and consequently quicker functional recovery.

Metaphyseal Hip prosthesis

Consist of femoral stems designed for anchorage in the metaphyseal area. They can preserve all or part of the femoral neck and provide  minimum bone sacrifice with preservation of diaphyseal region, thus allowing for a possible future revision.

They are combined with a standard type joint head and an acetabular prosthesis normally used for total hip replacements.

They may have a modular neck that ensures a more accurate reproduction of the anatomy and the physiological biomechanics.

Uni-compartmental knee prosthesis

Implants that replace only the damaged portion of the joint due to degenerative disease, provide minimal bone removal and fully preserve the contralateral condyle and the ligamentous apparatus. They are generally used in OA knees and in the medial uni-compartmental of the varus knee.

It is composed of a femoral metal component, that covers the damaged condyle and from a polyethylene tibial component that covers the corresponding surface on the tibia (tibial hemi plate). The tibial component can also be composed of two elements: a metal plate (metal-back) to be fixed to the bone on which a joint insert is fixed.

 

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