**2.11 Clicking and squeaking**

With the aid of fluoroscopy, articular surfaces separation during the hip motion has been observed. During this hip separation there is a loss of contact area, leaving only edge contact. Although separation has been well documented, it has not been correlated to clinical complications nor has a more in-depth understanding of the cause and effect been developed. Glaser et al studied the correlation between hip separation and sound production in different bearing surfaces.(Glaser 2008) In their study, among all analyzed subjects, the patients with a metal-on-polyethylene articulating surface experienced the highest magnitude of separation. The ceramic-on-polyethylene group had, on average, relatively low separation values, and these subjects demonstrated much less jerky motion. The maximum separation for the metal-on-metal polyethylene sandwich subjects was 1.5

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 177

same time period. Despite the recognized success and worldwide acceptance of total hip arthroplasty, wear of the UHMWPE component is a major obstacle limiting the longevity of

Fig. 6. Different current bearing surface options; A, metal-on-polyethylene, B, metal-on-

The metals used in conjunction with polyethylene principally have included stainless steel, cobalt-chromium alloy (in the vast majority), and titanium alloy. In some cases, the metal components have been surface hardened; for example, by nitriding or ion-implanting. In general, the wear rate of polyethylene against stainless steel has been comparable to that against cobalt-chrome alloy in laboratory tests and in clinical use. In contrast, although the wear rate of polyethylene against titanium alloy under clean conditions appears to be comparable to that with the other metals, the greater vulnerability of titanium alloy to abrasion by entrapped third-body particles can cause severe, runaway wear. Hardening of the surface of the titanium alloy by techniques such as gas nitriding, solution nitriding, or ion implanting can markedly improve its resistance to abrasion by third-body particles, and good 10-year results have been reported for titanium nitride-hardened TiAlNb alloy balls used with polyethylene cups. Nevertheless, if a hardened surface eventually is penetrated, severe wear of the underlying alloy still can be triggered. Consequently, even hardened titanium alloys have seen limited clinical use as bearing surfaces. The vast majority of metalon-polyethylene bearings used in hip prostheses have involved cobalt-chrome alloy femoral balls, including cast or forged alloys, and the wear rate of this combination now forms the clinical baseline against which potentially improved bearing combinations are evaluated. As

these reconstructions.

metal, and C, ceramic-on-ceramic

**3.1 Materials bearing against polyethylene 3.1.1 Metal-on-polyethylene bearings** 

mm. Interestingly, in the metal-on-metal group with large femoral heads (52 mm), all patients experienced very high separation values after heel-strike in the first 30% of stance phase, which decreased to a no-separation condition during midstance. In the metal-onmetal group with small-size femoral heads (38 mm), the trend was similar to the other bearings with small femoral heads.(Komistek 2002) For all patients, the sound signals were examined and compared with the kinematic findings. Interestingly, there was a distinct correlation of a high-frequency sound occurring at the time when the femoral head slid back into the acetabular component. As the femoral component impacted the acetabular cup, the sound sensor revealed a high-frequency sound, representing impact conditions. A thud-like "clicking" sound was detected for the subject having a metal-on-polyethylene bearing. Similar, but much more accentuated, was the sound recorded for the subjects with a ceramic-on-polyethylene prosthesis. Clear and rich "clicking," combined with some crepitus, was observed for the subjects having a metal-on-metal polyethylene sandwich total hip arthroplasty. The subjects with a metal-on-metal total hip prosthesis experienced a sound similar to a "rusty door hinge". Ceramic-on-ceramic total hip arthroplasty subjects experienced a "squeaking" sound of variable degree, which was present throughout the entire gait cycle. The ceramic-on-ceramic articulations were considered to be the noisiest.(Rodriguez 2008) In all patients with separation, a knocking sound was observed when the femoral head contacted the acetabulum.(Glaser 2008)

Mismatched ceramic couples, acetabular component malposition and impingement have been proposed as factors in the development of squeaking. However, not all mismatched and malpositioned components lead to squeaking. (Restrepo 2008) Additionally, squeaking has been observed in properly matched and positioned implants and when no evidence of neck-to-rim impingement is present. Audible squeaking of a hip replacement remains a still unexplained phenomenon. The sliding motion within the acetabular cup could lead to the induction of vibrational propagation across the interface of the femoral head and acetabular cup, possibly leading to audible interactions. It is also hypothesized that the "squeaking" sound, which mainly occurs in ceramic-on-ceramic total hip prostheses, may be due to separation of the femoral head. This movement of the femoral head from the acetabular component and the impact conditions generated from sliding back can be a source for the acoustic emission observed in total hip prostheses. (Jarrett 2009)
