The Ever So Hip Hip Implant

Total hip implants have made their way into mainstream media of late. Such attention has been raised because one company that focuses on hip replacement solutions, Depuy, is facing the grim reality that 12.5% of its hip implants (ASR XL or ASR Hip Resurfacing Systems) has failed according to an August 2010 study.  The  news for DePuy's parent company, Johnson & Johnson, seems to get worse when one considers the staggering number of U.S. citizens (100,000) that received the implant between 2005 and 2010.   Johnson & Johnson has set the stage for a massive voluntary recall of the two implant systems in which they will pick up the costs for monitoring, clinical visits, revision operations, and follow up.

With hip replacement surgeries having been around since 1960, advancements in surgical techniques and equipment have improved the outcomes for most patients to its highest levels.  Currently 193,000 hip replacements occur every year in the United States, making it one of the more common orthopedic procedures.  

Having said this, I'd like to take a step back from the main stream media's focus in this issue, and discuss the anatomical considerations for a total hip replacement, what indications are considered,  and how an implant works.

ANATOMY

The  hip is a ball and socket joint which is made up respectfully of the head of the femur bone (femoral head) and the acetabulum, a bony concave surface that dives into the pelvis.  With these two boney structures so closely related, resistance to movement can occur in the form of friction if they were to become in direct contact (see Problem Hip.)  To decrease this, the synovial membrane lubricates the space between these structures to help promote free movement while articular cartilage assists in cushioning the joint.

As the hip joint serves as one of the heaviest weight bearing joints in the body, its structural integrity is of infinite importance in day-to-day movements.  A few associated structures that help ensure this include:

• Capsule: located and attached just outside the acetabular rim is a fibrous lip that surrounds the neck of the femur.  Its function is to help promote a wide range of motion while assisting in weight bearing activities.
• Ligaments are a fibrous dense tissue that form a connection between bones and play a major role in helping solidify the integrity of a joint.  In our case of the hip, we have two main ligament groups that help reinforce:
1. Extracapsular: includes three ligaments that strengthen the capsule and prevent excessive range of motion in the joint.
2. Intracapsular: think inside the capsule. The main function here is to prevent further displacement of the femoral head in the event of a hip dislocation.

With an intact hip, the lower limbs are permitted to do four main movements:

Hip Abduction

          Hip Extension

With this proper functioning system in mind, a lot of things can go wrong that would lead a patient to need hip implant surgery.  Research has shown that 85% of patients with total hip replacement have a diagnosis of osteoarthritis (the most common joint abnormality) or a form of inflammatory arthritis (mainly rheumatoid arthritis.)  Let's examine these two factors in our investigation of the Total Hip Replacement.

OSTEOARTHRITIS (OA)

In this disease, the joint becomes compromised by years of “wear and tear.”  Think of the joint as becoming worn down and losing it's mechanical efficiencies with degradation in cartilage, bone and a build up of fibrous tissue. This increase in mechanical stress will come at a cost with increased friction and resistance to movement. Major causes of OA include a family history of osteoarthritis, being overweight, trauma to the joint itself, and long term over use.  

Symptoms of OA include deep aching joint pain that gets worse after activity and relieved with initiating rest.  As the condition progresses, pain becomes more prevalent during rest and eventually does not subside.  Additionally, patients may suffer from limited movement, grating (cracking) of joint on movement, muscle weakness around the joint, and joint swelling.  

A physical exam can demonstrate grating, joint swelling, limited range of motion, tenderness when pressed and pain with normal movement. 

RHEUMATOID ARTHRITIS (RA)

RA is considered of autoimmune origin.  Normally the immune system recognizes foreign or impaired self cells and attacks them to prevent their harmful effects.  In RA, the immune system recognizes normal cells as being foreign invaders and attacks and attempts to eliminate them.  RA can affect other joints and organs and generally does so bilaterally unlike osteoarthritis.   

Symptoms of RA include:

• Fatigue loss of appetite
• Morning stiffness
• Generalized muscle aches and weakness
• Unlike OA, as the joint becomes more used and “warmed up” the pain subsides.  

INDICATIONS FOR SURGERY:

Knowing these two main causes of degeneration of this joint, we can begin to understand the clinical picture presented to the orthopedic surgeon and how the recommendation is made for total hip replacement.  

Generally speaking, the severity of symptoms and degree of disability will dictate the indication.  Pain that has advanced into previous times where it was non-existent (during rest in OA; during activity in RA) plays an important factor.  

Functional limitations are also a key component to the surgical indication formula, but alone are not an indication for surgery.  With OA, walking and stair climbing become increasingly difficult with increased pain and decreased range of motion. Simple daily routines such as putting on one's shoes become massive hurdles in the course of the day.  In RA, these functional limitations are even more exaggerated and occur bilaterally more often than OA, which is more a unilateral clinical picture.  

Both these symptoms of pain and functional limitations are improved with surgery and the patient can often see a return to previous 'normal' levels in as little as three months post operatively.  With this understanding of the environment and indications for surgery, tomorrow we will discuss how a total hip replacement works.