The history and development of the Exeter Hip

Due largely to the work of four English Orthopaedic surgeons: Sir John Charnley of Manchester, G.K.McKee and J. Watson-Farrar of Norwich and Peter Ring of Redhill, Surrey, total replacement of the hip had become, by the mid- nineteen sixties, a practical proposition for the general orthopaedic surgeon. This operation produced a revolution for both surgeon and patient alike. Instead of weeks in hospital and an uncertain result from the surgical treatment of arthritis of the hip, a common and crippling disorder, total replacement appeared regularly to achieve a painless functioning hip with no more than 10-14 days as an in-patient. The inevitable consequence was an huge increase in demand so that the operation rapidly spread  throughout the U.K. and subsequently, the world. In some countries, demand for the procedure soon swamped the available orthopaedic facilities.

Gradually, it became clear that, although the initial results were spectacular, a number of patients later developed problems with their replaced hips due to loosening of the prosthetic components from the bone, infection, dislocation or breakage of the implants themselves. Research into the causes of these complications became widespread and led to the introduction of new designs and materials that did not always turn out to fulfil the optimism with which they were introduced. Such failures demanded re-operation and further increased the pressure on the surgical facilities.

In Exeter, starting in April, 1965, the initial experience of total hip replacement was with the cemented McKee-Farrar prosthesis, in which both the artificial ball and the socket were made of metal – the so-called ‘metal on metal’ bearing. This type of bearing was also used in the uncemented Ring hip, whereas the cemented device introduced by Sir John Charnley utilised a metal ball and a plastic socket, initially made of teflon and subsequently high molecular weight polyethylene, the ‘metal on  plastic’ bearing. Over the following four years, a significant number of the McKee-Farrar hips inserted in Exeter failed because of loosening, that was attributed to the unfavourable frictional behaviour of the ‘metal on metal’ bearing and became such a problem that it was decided to abandon the use of this implant. The question then arose of which device to use in its stead. The ‘metal on metal’ issue precluded the Ring implant and consideration was given to adopting the Charnley hip. However, a required and reasonable part of the technique for the use of the Charnley  involved the lateral surgical approach to the hip, a complex procedure involving the removal and subsequent re-attachment of part of the upper end of the femur (thigh bone) with its muscle attachments. None of the Exeter surgeons were willing to adapt this method since they had become familiar with the simpler posterior approach.

(fig.2)

The requirement for a ‘metal on plastic’ cemented hip suitable for use through the posterior approach led to the development during 1969 of what subsequently became known as the Exeter Hip. Professional engineering input was sought in the School of Engineering Science at the University of Exeter and after pre-clinical laboratory testing, the new design was first implanted in November, 1970. The design of the new device (Fig. 1) was unique in two particular respects: the femoral stem (that carries the ball of the hip joint) was totally devoid of any sort of neck collar (see Fig. 2 that shows the large collar of the American T28 stem),  hitherto an invariable feature of previous designs, in some of which it was intended to transmit the hip joint loads directly into the cut surface of the femoral neck. Many believed at the time that such collars were essential for adequate mechanical loading of the upper part of the thigh bone, without which the latter would atrophy. However, in Exeter, the regular appearance in follow-up X-rays of the (collared) Mckee-Farrar implant of resorption (i.e. disappearance) of the bone of the  neck under the collar showed that the latter was not important for direct effective loading of the femur and justified the removal of the collar in the new design. The second unique feature was the double taper configuration throughout the whole length of the stem. This was adopted primarily to improve the ability of the stem to force the doughy acrylic bone cement into the bone of the femur whilst the stem was being inserted into the canal of the femur and so improve the fixation.

(fig.1)

Since these two main features of the new design were radical departures from what had hitherto been current practice, the manufacturers of the device (The London Splint Company) responsibly agreed that its use should be limited to surgeons in Exeter or those who had been trained in Exeter until at least five years of clinical experience with the device was available.

After the new implant had been in use for eighteen months, with entirely satisfactory clinical results, an unexpected finding emerged on follow-up X-rays. These showed that the stem was subsiding slightly within its surrounding bone cement. This phenomenon had not at that time been reported with any other type of implant and was initially a source of some anxiety. Gradually, however, it became apparent that such subsidence was almost always entirely benign and not associated with loosening of the implant – almost the opposite, in fact, since loosening of the original Exeter stems proved to be very rare. Follow-up studies of the original group of Exeter hips into the 33^rd year since operation show a re-operation rate for stem loosening of only 3.46% (an extremely low figure), in spite of the fact that surgeons of widely differing experience were involved in the surgery and the cementing techniques then in use were, by modern standards, crude in the extreme.

What did emerge, however, were a number of stem breakages (3.92% to date) and by 1974 it was clear that a stronger stem was required, together with a greater range of sizes. The original stems were manufactured from the stainless steel alloy EN58J, the British Standard for which demanded that any implant made from this alloy should be given a polished surface. The new range of stems (Fig. 3), introduced at the beginning of 1976, were heavier in section than the original stems and manufactured from 316L stainless steel that was stronger and less ductile than EN58J and for which there was no standard demanding a polished surface. Since  the polishing process is expensive, the new stems were manufactured with a non-polished (matt) surface that was actually two orders of magnitude (i.e. 100 times) rougher than the surface of the original polished stems. At the time, nearly all types of femoral stem were manufactured with a matt surface. Over the next few years, the paradoxical finding gradually emerged that  whilst the new, matt-surfaced Exeter stems virtually obliterated breakage as a complication, their loosening rate was substantially higher than with the original polished Exeter implants.

Since the main change from the original polished Exeter to the matt-surfaced version was the change in surface finish, it naturally seemed that the surface changes were likely to be responsible for the increased loosening rate of the matt-surfaced stems. However, clear evidence that this was the case was not obtained until studies of retrievals of the loosened matt-surfaced stems revealed  evidence of major abrasive wear on their surfaces. This process was evidently associated with the production of much wear debris and gradual attenuation of the inside of the cement where it had worn away through  abrasion against the rough stem. These mechanisms resulted in gradual loosening of the stem in some 10% of cases by 10 years of follow-up. Such abrasive wear was not seen on the few retrieved polished stems that were examined. 

(fig.3)

The solution to this loosening problem seemed clear: the stem surface would have to be polished again. This change was made in 1986. Two years later, the polished Exeter Universal stem was introduced (Fig.4) and has been in regular use ever since. This device retained the collarless, double-tapered design that has been common to all Exeter stems, and incorporated a modular system that allowed the use of different head sizes and bearing combinations. The high fatigue strength, low corrosion stainless steel alloy REX 734 was used for its manufacture and the range of available stem sizes has been gradually extended to suit all comers.  In Exeter, with modern cementing methods, in-part developed in Exeter,  the Exeter Universal stem has proved to be extremely reliable over the last  15 years in all age groups,  in the hands of surgeons of widely differing experience, and associated with notably benign X-ray appearances. It is similar to but much stronger than the original polished Exeter stems and with the use of modern methods of cementing, there is every reason to expect it to function satisfactorily for 30 years and more.

(fig.4)

With progressively longer follow-ups, together with extensive laboratory studies of the viscoelastic properties of cement performed in the School of Engineering and Computer Science at the University of Exeter, the basis for the unusually reliable function of the polished Exeter stem has gradually become clear and is related to the small degrees of subsidence of the polished, double tapered Exeter stem within the mantle of cement. The stem is acting as an engineering taper with respect to the cement. Tapers are used frequently in engineering practice for the reliable transmission of axial and torsional loads. In the context of  the Exeter stem, the subsidence and the taper action, combined with the visco-elastic properties of the cement, are responsible for generating a loading regime that is dominated by compression (well tolerated by the cement and bone) and reduces damaging shear forces. This improves the torsional stability of the stem within the femur, an extremely important factor in the long-term function of hip replacement. Moreover, such a loading regime is associated with unusually good preservation of the proximal bone of the femur, another factor likely to contribute to good very long term results.

Such behaviour is unique to the polished Exeter stem design that has now become very widely used in many countries.