Insights into Imprinting: How Is the Phenomenon of Tribocorrosion at Head-Neck Taper Interfaces Related to Corrosion, Fretting, and Implant Design Parameters?

Abstract

Background: Wear and corrosion at modular neck tapers in THA can lead to major clinical implications such as periprosthetic osteolysis, adverse local tissue reactions, or implant failure. The material degradation processes at the taper interface are complex and involve fretting corrosion, third-body abrasion, as well as electrochemical and crevice corrosion. One phenomenon in this context is imprinting of the head taper, where the initially smooth surface develops a topography that reflects the rougher neck taper profile. The formation mechanism of this specific phenomenon, and its relation to other observed damage features, is unclear. An analysis of retrieved implants may offer some insights into this process.

Questions/purposes: (1) Is imprinting related to time in situ of the implants and to the taper damage modes of corrosion and fretting? (2) Are implant design parameters like neck taper profile, stem material, or head seating associated with the formation of imprinting? (3) Is imprinting created by an impression of the neck taper profile or can a different mechanistic explanation for imprinting be derived?

Methods: Thirty-one THAs with cobalt-chromium-molybdenum-alloy (CoCrMo) heads retrieved between 2013 and 2019 at revision surgery from an institutional registry were investigated. Inclusion criteria were: 12/14 tapers, a head size of 36 mm or smaller, time in situ more than 1 year, and intact nonmodular stems without sleeve adaptors. After grouping the residual THAs according to stem type, stem material, and manufacturer, all groups of three or more were included. Of the resulting subset of 31 retrievals, nine THAs exhibited a still assembled head-neck taper connection. The median (range) time in situ was 5 years (1 to 23). Two stem materials (21 titanium-alloy and 10 stainless steel), three kinds of bearing couples (11 metal-on-metal, 13 metal-on-polyethylene, and seven dual-mobility heads), and two different neck taper profiles (six wavy profile and 25 fluted profile) were present in the collection. Four THAs exhibited signs of eccentric head seating. The 31 investigated THAs represented 21% of the retrieved THAs with a CoCrMo alloy head during the specified period.At the head tapers, the damage modes of corrosion, fretting, and imprinting were semiquantitatively rated on a scale between 0 (no corrosion/fretting/imprinting) and 3 (severe corrosion/fretting/imprinting). Corrosion and fretting were assessed applying the Goldberg score, with the modification that the scale started at 0 and not at 1. Imprinting was assessed with a custom scoring system. Rating was done individually at the proximal and distal head taper half and summed to one total damage score for each retrieval and damage mode. Correlations between the damage modes and time in situ and between the damage modes among each other, were assessed using the Spearman rank order correlation coefficient (ρ). Associations between imprinting and implant design parameters were investigated by comparing the total imprinting score distributions with the Mann-Whitney U-test. Metallographically prepared cross-sections of assembled head-neck taper connections were examined by optical microscopy and disassembled head and neck taper surfaces were assessed by scanning electron microscopy (SEM).

Results: The imprinting damage score increased with time in-situ (ρ = 0.72; p < 0.001) and the corrosion damage score (ρ = 0.63; p < 0.001) but not with the fretting damage score (ρ = 0.35; p = 0.05). There was no difference in total imprinting score comparing neck taper profiles or stem materials, with the numbers available. Eccentric head seating had elevated total imprinting score (median 6 [interquartile range 0]) compared with centric seating (median 1 [2]; p = 0.001). Light optical investigations showed that imprinting can be present on the head taper surfaces even if the depth of abraded material exceeds the neck taper profile height. SEM investigations showed bands of pitting corrosion in the imprinted grooves.

Conclusion: The microscopic investigations suggest that imprinting is not an independent phenomenon but a process that accompanies the continuous material degradation of the head taper surface because of circular damage on the passive layer induced by grooved neck tapers.

Clinical relevance: Material loss from head-neck taper connections involving CoCrMo alloy heads is a source of metal ions and could potentially be reduced if hip stems with smooth neck tapers were used. Surgeons should pay attention to the exact centric seating of the femoral head onto the stem taper during joining of the parts.

Fig. 1

Flow chart of the experimental approach.

Fig. 2

Light optical images (upper row: top view; bottom row: cross-section) of the two surface profile types present in the investigated retrieval collection. The left side refers to the wavy neck taper profile of the Smith & Nephew implants, and the images on the right refer to the fluted surface profile of the Zimmer implants.

Fig. 3

A-B Retrieved CoCrMo heads showing different degrees of imprinting. (A) The proximal contact zone was rated with a score of 1, and the distal contact zone did not show signs of imprinting (score 0). (B) CoCrMo head with severe imprinting in the proximal and distal contact zones (score 3) is shown.

Fig. 4

Correlation between the total damage score for imprinting and the time in situ (n = 30) as well as total corrosion (n = 31) and fretting scores (n = 31), respectively. The data points are discriminated with respect to the bearing couples; MoM = metal-on-metal; MoP = metal-on-polyethylene; DM = dual-mobility head.

Fig. 5

Distribution of the severity of the damage modes of corrosion, fretting, and imprinting of the retrieved heads.

Fig. 6

Box plots illustrating the distribution of the damage scores in the distal and proximal halves of 31 retrieved head tapers. Squares represent the respective mean values, diamonds data outliers.

Fig. 7

Box plots illustrating the distribution of the imprinting damage score for the retrievals grouped according to stem material, neck taper profile, and head seating for the collection comprising 23 retrievals (no DM heads). Squares represent the respective mean values.

Fig. 8

A-C Light optical images of retrieved head tapers with signs of eccentric head positioning. (A-B) These figures refer to the same retrieval; (A) shows the head taper before and (B) shows the head taper after cleaning. The white arrow points to the imprinted surface area, and the black arrow indicates the original head taper surface topography. (C) A head taper that has been cut for metallographic sample preparation. The areas encircled by dotted lines refer to the gaps resulting from eccentric head seating.

Fig. 9

A-D Light optical images of a cross-section of the head-neck interface of a retrieval with distinct imprinting. The neck taper exhibits the fluted surface profile. (A) The gap between neck and head taper resulting from eccentric head seating is shown. (B) The plastically deformed neck taper surface profile is shown. No imprinting ocurred here. (C) The image shows the depression at the head taper in the distal contact region and the abraded neck taper profile. (D) The image represents the area with distinct imprinting at the head taper surface. A color image accompanies the online version of this article.

Fig. 10

A-E Light optical images of a cross-section of the head-neck taper interface of a retrieval with distinct imprinting and wavy neck taper profile. (A) The image shows the generated depression at the distal end of the taper contact zone. (B) The image shows the imprinted head taper surface. (C) The image shows the generated depression at the proximal end of the contact zone as well as imprinting at the head taper. (D) The image shows the gap between head and neck. (E) The figure shows slight alterations in surface topography at the head and the stem taper surface.

Fig. 11

A-C Scanning electron microscopy images of a head taper with severe imprinting. (A) The generated surface exhibits a corrugated topography. In between the generated peaks, bands of pitting damage are present. (B) The arrows indicate observed debris. The white square in Fig. 11B indicates the location of image Fig. 11C. (C) The image shows the observed bands of pitting damage at a higher magnification.

Fig. 12

A-B Scanning electron microscopy images of a head taper with signs of moderate imprinting. (A) The marks generated by the imprinting process are indicated by white arrows. In between the marks, the original surface topography of the head taper is still present. The square in Fig. 12A indicates the location of Fig. 12B. (B) The marks are characterized by abrasive damage.