Joint Replacement

I saw a longstanding patient of mine last week for a catch up regarding his hips. I hadn’t seen him for about 3 years. He had just celebrated his 95^th birthday and is still living independently and very active. He told me he had retired from playing tennis a couple of years ago at 93! I had operated on him 10 years ago when he had a problem with his right hip replacement. He brought along some xrays of his hips. His right side was fine and his left hip showed a hip replacement with signs of significant socket (acetabular) wear but he had no symptoms at all. How old is that hip replacement ? – 20 years? Or more??

Figure 1. An original cemented Charnley Hip Replacement – the so called Low Friction Arthroplasty. Note the eccentric (off- centre) position of the femoral head (ball) in relation to the wire marker of the socket indicating significant socket polyethylene surface wear.

Well, his left hip was replaced in 1974 by a Melbourne surgeon, now retired. He has an original Charnley cemented one piece femoral stem/head and acetabulum undertaken through a direct lateral approach with trochanteric osteotomy, as per the operative technique described by Charnley for his Low Friction Arthroplasty (LFA). We would consider this LFA nowadays to be the prototype of all modern hip replacement implants, undertaken using what would now be considered fairly primitive cementing and surgical techniques. It is 42 years old and aside from the wear, the implant itself looks solidly fixed to bone. An amazing result for a metal on polyethylene bearing, particularly given that the Charnley LFA is a small metal head (22.2mm stainless steel head) articulating with an (unadulterated, non-crosslinked) all polyethylene socket. This hip replacement was undertaken in a man of 53 years who has led an active life including playing sport – certainly a very good test for any hip replacement and its articulating surfaces. This got me thinking about joint replacement bearing surfaces. What is the best bearing surface and particularly what is the best bearing surface for a younger active patient undergoing hip replacement?

Figure 2. Socket bearing surfaces in hip replacement – metal (left), ceramic (middle) and polyethylene (right).

The reality is – there is no ideal surface, just advantages and disadvantages to each coupling. An ideal bearing surface would be one where the wear is minimal and the wear debris particles generated cause no local or systemic problem. If such a surface was available then joint replacements could last the term of one’s natural life, irrespective of activity level.

There are multiple factors influencing implant longevity. The first of these is naturally the age and life expectancy of the recipient. The activity level and weight of the patient are very significant factors. Traditionally young active patients have a far greater chance of requiring revision hip replacement surgery. The implant itself is a factor but questionably the most important factor may be the choice of implant bearing surface. The alignment of the bearing surfaces at the time of surgery will influence longevity – it has been well documented that poor alignment leads to edge loading and excessive wear on any surface.

Tribology is the science associated with the study of surfaces interacting under load and incorporates the concepts of friction, wear and lubrication. The pursuit of excellence in bearing surface technology has led to a number of alternative bearing surface couplings being available in total hip replacement surgery. The surfaces commonly used include “soft” bearings such as metal or ceramic on polyethylene and “hard” bearings like metal on metal or ceramic on ceramic.

Figure 2. Wear rates of different hip replacement bearing surfaces.

Metal on poly (MoP) is the most commonly used surface in hip replacement surgery. Modern day polyethylene is generally cross-linked and this has reduced surface/particulate wear to about 10% of that seen with original untreated poly. MoP surfaces are durable, versatile and have a long, successful and well researched history. The particulate wear is not toxic but can be associated with osteolysis and late aseptic loosening. Ceramic on poly has slightly less wear than MoP but does have a bedding in period and some issues associated with ceramics.

Metal on metal (MoM) surfaces have been around since the 60’s with the McKee Farrar and Ring prostheses. They had issues with with high rates of loosening, being largely abandoned in the late 70’s. MoM underwent a resurgence in the late 90’s with both conventional and resurfacing type hip replacements. These surfaces are generally made of a highly polished cobalt/chromium alloy. MoM interfaces show low rates of wear compared with MoP, but despite this, the number of particles generated each year is about 14,000x more than an equivalent polyethylene surface. These tiny sub-micron particles accumulate in the bone and soft tissue surrounding the implant and in susceptible individuals can lead to a hypersensitivity tissue reaction. If this occurs early aseptic loosening of the implant ensues often with significant bone and soft tissue destruction or pseudotumour formation. In some patients increased serum levels of cobalt and chromium ions can reach toxic levels with deposition in liver, kidney and central nervous systems and unknown long term effects.

Ceramic on ceramic (CoC) articulations are potentially high performing surfaces with the least wear of any bearing surface. Similar to polyethylene, particulate debris is not generally toxic but can still be associated with aseptic loosening. Ceramic does have an incidence of fracture with potentially sudden catastrophic failure and is associated with up to a 20% incidence of implant “squeaking” with movement. Poorly sited CoC implants with edge loading can lead to “chipping” and the generation of large ceramic shards or fragments. If a CoC surface needs to be revised for any reason it is generally accepted that the same surface needs to used again. This is because of the potential abrasiveness of ceramic particles creating 3^rd body wear (accelerated wear from particles inside an articulation scratching the surface with movement).

In general, “hard” bearings wear less and should last longer but are more expensive and less forgiving. They require more precise implant placement to reduce impingement and edge loading and risk of fracture. “Soft” bearings are more forgiving and cross-linked polyethylene now has excellent clinical results out to nearly 20 years.

I’m pretty conservative in my practice and almost exclusively use metal on x-linked polyethylene for all patients, including younger more active individuals undergoing hip replacement surgery. It is the bearing surface I feel most comfortable with. It has excellent longevity, is unquestionably the safest articulation and if revision surgery is required it does not limit future alternative bearing surface options.

What is the low-down on joint replacements and airport security? Some of the questions that I get frequently asked are – Is my hip/knee replacement going to set off metal detectors at the airport? Will I be detained or searched? Do I need a letter or card stating I have a hip/knee replacement?

Following the World Trade Centre terrorist attacks in 2001, airport security checks have significantly increased. Amongst a number of measures, travellers and their travel-on luggage and are now generally screened by metal detectors. People with joint replacement implants will often activate metal detectors. If you are in this position, what should you do?

Well the first thing is don’t panic and just be prepared for some increased scrutiny. It may take a little more time (usually not more than a minute or so) to clear security. Cards or letters indicating that you have a joint replacement can be helpful but won’t allow you to bypass the usual security measures. Indicating to the security staff that you have a joint replacement before entering the scanners is helpful and may save having multiple attempts at going through the detectors. Almost certainly a body sweep with a hand held detector wand will indicate to staff the presence of a hip or knee replacement. Some countries now have total body scanners that use either a microwave type or back scatter technology that allows for direct visualization of any internal implant. Occasionally if security staff are not satisfied a manual “pat down” search may be required.

Interestingly a number of patients have told me that their implants were not detected at certain airports, but identified at others. The sensitivity of various metal detectors clearly varies and implants made of predominantly non ferrous metals like titanium are not always picked up.

The key thing at airport security is to stay calm and be polite. A confrontational approach with security staff generally doesn’t end well – bear in mind that they are doing their best to ensure the safety of all travellers. Being helpful and co-operative will generally see you on your way without undue delay.

I attended our regular journal club meeting for March 2016 where a number of interesting papers focusing on joint replacement and knee surgery were discussed.

The most intriguing paper of the evening was one looking at the old “chestnut” topic of computer navigated versus conventional Total Knee Replacement (TKR) – which is better? This study out of South Korea had a number of unique design features and importantly a disclosure statement indicating no conflict of interest (no relationships with and no funds received by any implant company) – quite unusual for a study of this size.

The study was a single surgeon series of an amazing 520 patients who each underwent bilateral total knee replacement (1040 knees studied) at a single institution. With each surgery one knee was randomly assigned to computer navigated knee replacement whilst the other underwent a conventional TKR procedure – a controlled trial of technique in each patient!

Interestingly 85% of the patients were female, with an average age of 68 years and an average BMI of 27 – a slightly different demographic to our standard Australian population undergoing TKR.

These patients were followed prospectively for an average of 10.8 years with clinical (Knee Society, WOMAC and UCLA activity scores), radiological (long leg alignment xrays and CT) assessment and survivorship analysis being undertaken.

A number of authors in the past have espoused the advantages of computer navigated over conventional TKR with regard to implant alignment and unrelated retrospective studies have suggested that an optimal “window” of alignment (3 degrees either side of neutral mechanical alignment) corresponds with better long term implant survivorship.

This paper indicated that in this quite unique cohort of patients, there was no difference in clinical scores for pain and function, no difference in implant alignment radiologically in either coronal and rotational planes and most importantly no difference in survivorship of implant between conventional technique and computer navigated TKR groups over a 10 year follow up period.

The strengths of this study were the large number of TKRs studied, the fact that all procedures were performed by a single surgeon (allowing reliability of technique) and most importantly some medium to long term follow up both clinically and radiologically. The weaknesses of the study were that interpretation of pain and function scores would not necessarily select out one knee group over another and this cohort of patients is demographically quite different to a Western society population undergoing TKR. It would have been really interesting (but not necessarily scientific) if patients were asked which knee they preferred or felt was better and to see if there was a difference between the groups.

Overall a really interesting paper that challenges beliefs held in some quarters, that computer navigation leads to superior results. There is no question that navigation increases operative time and there is morbidity associated with pin insertion. If preliminary landmark registration is not meticulous and the arrays are not protected from movement, computer navigation can lead to suboptimal implant placement and alignment.

Ultimately there is no best or superior way to undertake total knee replacement. Straightforward cases can be performed well by any experienced surgeon with conventional instrumentation and techniques. There are undoubtedly difficult cases, particularly those with significant bony deformity around the knee, where computer navigation or now patient specific instrumentation can be beneficial to both patient and surgeon.

Source: Computer-Navigated Versus Conventional Total Knee Arthroplasty. J Bone Joint Surg Am. 2012; 94: 2017-24.