Søren Overgaard

Resorption of hydroxyapatite and fluorapatite ceramic coatings on weight-bearing implants: A quantitative and morphological study in dogs

Resorption (defined as loss of ceramic coating because of cellular activity or dissolution) of ceramic coatings is a matter of concern for the long-term performance of ceramic-coated implants. A new fluorine-containing coating, fluorapatite (FA), has been shown to be more stable than hydroxyapatite (HA) in unloaded models. In a weight-bearing model in trabecular bone, we evaluated loss (defined as reduction of coating irrespective of type of mechanism) of HA and FA coatings during 25 weeks of implantation. Eight mature dogs had HA- or FA-coated implants inserted bilaterally into the weight-bearing region of the medial femoral condyle. Quantified loss of ceramic coating was estimated at the light microscopic level using stereological methods. The experiment showed significant loss of both types of coatings. However, no statistical difference in loss of ceramic coating was found regarding surface area implant coverage, volume, and thickness (p = 0.77, p = 0.13, p = 0.56, p = 0.23, respectively). Completely resorbed HA coating was replaced by 36 +/- 6.0% (range: 26-42) bone in direct contact with the implant surface compared with 29 +/- 16.0% (range: 12-59) for FA (p = 0.40), suggesting that the implant was firmly fixed despite loss of the ceramic coating. Transmission electron microscopy in combination with electron energy spectroscopy and electron spectroscopic imaging showed that osteclast-like cells, osteocytes, macrophage-like cells, and fibroblasts had phagocytosed calcium-containing fragments, indicating cell-mediated resorption of the ceramic coating.

Hydroxyapatite and fluorapatite coatings for fixation of weight loaded implants.

Survivor analysis of total hip replacement recently has shown disappointing results in younger patients. To improve this, ceramic coatings have been applied to prostheses for cementless use. A new fluorine containing coating, fluorapatite, has been shown to increase bone ingrowth compared with hydroxyapatite in unloaded models. In a weight loaded model, the effects of hydroxyapatite and fluorapatite coated implants on implant fixation and bone ingrowth were evaluated. Eight hydroxyapatite and fluorapatite coated implants with porous surface were inserted into the medial femoral condyles of 8 mature dogs in a paired design. The implants initially were surrounded by a gap communicating with the joint space and were loaded during each gait cycle. After 25 weeks, no differences in pushout data or bone ingrowth between hydroxyapatite and fluorapatite coated implants were found. An important finding was the absence of foreign body reaction in the bone. Neither hydroxyapatite nor fluorapatite coatings delaminated during implantation or as a result of the pushout test. Bone repair activity remained in the initial gap zone, but most of the bone was of the lamellar type. No difference in bone remodeling between the hydroxyapatite and fluorapatite coated implants was found in the initial gap zone. Microprobe analysis showed no increase in fluorine content around the fluorapatite coated implants. The hydroxyapatite and fluorapatite coatings seem efficacious after a 25-week implantation period under weight loaded conditions.

Inferior outcome after hip resurfacing arthroplasty than after conventional arthroplasty: Evidence from the Nordic Arthroplasty Register Association (NARA) database, 1995 to 2007

Background and purpose The reported outcomes of hip resurfacing arthroplasty (HRA) vary. The frequency of this procedure in Denmark, Norway, and Sweden is low. We therefore determined the outcome of HRA in the NARA database, which is common to all 3 countries, and compared it to the outcome of conventional total hip arthroplasty (THA). Methods The risk of non-septic revision within 2 years was analyzed in 1,638 HRAs and compared to that for 172,554 conventional total hip arthroplasties (THAs), using Cox regression models. We calculated relative risk (RR) of revision and 95% confidence interval. Results HRA had an almost 3-fold increased revision risk compared to THA (RR = 2.7, 95% CI: 1.9–3.7). The difference was even greater when HRA was compared to the THA subgroup of cemented THAs (RR = 3.8, CI: 2.7–5.3). For men below 50 years of age, this difference was less pronounced (HRA vs. THA: RR = 1.9, CI: 1.0–3.9; HRA vs. cemented THA: RR = 2.4, CI: 1.1–5.3), but it was even more pronounced in women of the same age group (HRA vs. THA: RR = 4.7, CI: 2.6–8.5; HRA vs. cemented THA: RR = 7.4, CI: 3.7–15). Within the HRA group, risk of non-septic revision was reduced in hospitals performing ≥ 70 HRAs annually (RR = 0.3, CI: 0.1–0.7) and with use of Birmingham hip resurfacing (BHR) rather than the other designs as a group (RR = 0.3, CI: 0.1–0.7). Risk of early revision was also reduced in males (RR = 0.5, CI: 0.2–0.9). The femoral head diameter alone had no statistically significant influence on the early revision rate, but it eliminated the significance of male sex in a combined analysis. Interpretation In general, our results do not support continued use of hip resurfacing arthroplasty. Men had a lower early revision rate, which was still higher than observed for all-cemented hips. Further follow-up is necessary to determine whether HRA might be useful as an alternative in males.

The influence of crystallinity of the hydroxyapatite coating on the fixation of implants: MECHANICAL AND HISTOMORPHOMETRIC RESULTS

We inserted two hydroxyapatite (HA)-coated implants with crystallinities of either 50% (HA-50%) or 75% (HA-75%) bilaterally into the medial femoral condyles of the knees of 16 dogs. The implants were allocated to two groups with implantation periods of 16 and 32 weeks. They were weight-bearing and subjected to controlled micromovement of 250 microm during each gait cycle. After 16 weeks, mechanical fixation of the HA-50% implants was increased threefold as compared with the HA-75% implants. After 32 weeks there was no difference between HA-50% and HA-75%. Fixation of HA-75% increased from 16 to 32 weeks whereas that of HA-50% was unchanged. HA-50% implants had 100% more bone ingrowth than HA-75% implants after 16 weeks. More HA coating was removed on HA-50% implants compared with HA-75% implants after both 16 and 32 weeks. No further loss of the HA coating was shown from 16 to 32 weeks. Our study suggests that the crystallinity of the HA coating is an important factor in its bioactivity and resorption during weight-bearing conditions. Our findings suggest two phases of coating resorption, an initial rapid loss, followed by a slow loss. Resorbed HA coating was partly replaced by bone ingrowth, suggesting that implant fixation will be durable.

Sealing effect of hydroxyapatite coating: A 12-month study in canines

This study addresses the clinical problems regarding access of wear debris to the bone-implant interface and the possible dissemination of polyethylene (PE) particles to distant organs. We inserted two implants into each knee of 7 dogs allowing access of joint fluid to the bone-implant interface with a 0.75 mm initial gap around the implant. Hydroxyapatite (HA)-coated and non-coated (Ti) titanium alloy implants were randomly allocated to each distal femoral condyle. PE particles were repeatedly injected into the right knee joint 3 weeks after surgery for a period of 49 weeks, while only vehicle was injected into the left knee joint. We found huge amounts of PE particles mainly in the bone-implant interface around Ti implants. Infiltration of mononuclear inflammatory cells was present around 3 of 7 Ti implants in relation to PE particles. HA implants had approximately 70% bone ongrowth. In contrast, no bone ongrowth was seen on any Ti implants, all being surrounded by a fibrous membrane. The number of PE particles was evaluated semi-quantitatively. More PE particles were found around Ti implants than with HA implants (p < 0.002). Specimens from iliac lymph nodes, liver, spleen and lung were examined and showed dissemination of PE particles only in regional lymph nodes.

Porous-coated versus grit-blasted surface texture of hydroxyapatite-coated implants during controlled micromotion

Hydroxyapatite (HA)-coated implants with porous-coated and grit-blasted surface textures were inserted bilaterally in a paired design into the medial femoral condyles of eight dogs for 16 weeks. The implants were weight-loaded and initially subjected to controlled micromotion of 500 microm during each gait cycle. Histology revealed that five implants in each group had bony anchorage, and the remaining implants were surrounded by fibrous tissue. Push-out testing showed no difference in shear stiffness and strength, while energy absorption for porous-coated implants was increased significantly by threefold. The HA coating delaminated on grit-blasted implants during push-out testing, whereas porous-coated implants predominantly failed at the HA-tissue interface. Coverage, surface area, volume, and thickness of the HA coating were significantly reduced in vivo for porous-coated and grit-blasted implants. In conclusion, a plasma-sprayed porous-coated implant surface seems to give better fixation not only of the HA-coating to the implant surface but also of the implant to the surrounding tissues in comparison to a grit-blasted implant surface. The HA coating was reduced more on fibrous-anchored than on bony-anchored implants, suggesting that micromotion accelerates resorption of HA. Resorbed HA coating was replaced by more bone on porous-coated implants than on grit-blasted implants, which suggests that fixation of porous-coated implants will be durable.

The effects of hydroxyapatite coating and bone allograft on fixation of loaded experimental primary and revision implants

We used our established experimental model of revision joint replacement to examine the roles of hydroxyapatite coating and bone graft in improving the fixation of revision implants. The revision protocol uses the Søballe micromotion device in a preliminary 8-week period of implant instability for the presence of particulate polyethylene. During this procedure, a sclerotic endosteal bone rim forms, and a dense fibrous membrane is engendered, having macrophages with ingested polyethylene and high levels of inflammatory cytokines. At the time of revision after 8 weeks, the cavity is revised with either a titanium alloy (Ti) or a hydroxyapatite (HA) 6.0 mm plasma-sprayed implant, in the presence or absence of allograft packed into the initial 0.75 mm peri-implant gap. The contralateral limb is subjected to primary surgery with the same implant configuration, and serves as control. 8 implants were included in each of the 8 treatment groups (total 64 implants in 32 dogs). The observation period was 4 weeks after revision. Outcome measures are based on histomorphometry and mechanical pushout properties. The revision setting was always inferior to its primary counterpart. Bone graft improved the revision fixation in all treatment groups, as also did the HA coating. The sole exception was revision-grafted HA implants, which reached the same fixation as primary Ti and HA grafted implants. The revision, which was less active in general, seems to need the dual stimulation of bone graft and HA implant surface, to obtain the same level of fixation associated with primary implants. Our findings suggest that the combination of HA implant and bone graft may be of benefit in the clinical revision implant setting.

Increased risk of revision in patients with non-traumatic femoral head necrosis.

Background and purpose Previous studies of patients who have undergone total hip arthroplasty (THA) due to femoral head necrosis (FHN) have shown an increased risk of revision compared to cases with primary osteoarthritis (POA), but recent studies have suggested that this procedure is not associated with poor outcome. We compared the risk of revision after operation with THA due to FHN or POA in the Nordic Arthroplasty Register Association (NARA) database including Denmark, Finland, Norway, and Sweden. Patients and methods 427,806 THAs performed between 1995 and 2011 were included. The relative risk of revision for any reason, for aseptic loosening, dislocation, deep infection, and periprosthetic fracture was studied before and after adjustment for covariates using Cox regression models. Results 416,217 hips with POA (mean age 69 (SD 10), 59% females) and 11,589 with FHN (mean age 65 (SD 16), 58% females) were registered. The mean follow-up was 6.3 (SD 4.3) years. After 2 years of observation, 1.7% in the POA group and 3.0% in the FHN group had been revised. The corresponding proportions after 16 years of observation were 4.2% and 6.1%, respectively. The 16-year survival in the 2 groups was 86% (95% CI: 86–86) and 77% (CI: 74–80). After adjusting for covariates, the relative risk (RR) of revision for any reason was higher in patients with FHN for both periods studied (up to 2 years: RR = 1.44, 95% CI: 1.34–1.54; p < 0.001; and 2–16 years: RR = 1.25, 1.14–1.38; p < 0.001). Interpretation Patients with FHN had an overall increased risk of revision. This increased risk persisted over the entire period of observation and covered more or less all of the 4 most common reasons for revision.

Hydroxyapatite coating does not improve uncemented stem survival after total hip arthroplasty!: An analysis of 116,069 THAs in the Nordic Arthroplasty Register Association (NARA) database

Background and purpose — It is still being debated whether HA coating of uncemented stems used in total hip arthroplasty (THA) improves implant survival. We therefore investigated different uncemented stem brands, with and without HA coating, regarding early and long-term survival. Patients and methods — We identified 152,410 THA procedures using uncemented stems that were performed between 1995 and 2011 and registered in the Nordic Arthroplasty Register Association (NARA) database. We excluded 19,446 procedures that used stem brands less than 500 times in each country, procedures performed due to diagnoses other than osteoarthritis or pediatric hip disease, and procedures with missing information on the type of coating. 22 stem brands remained (which were used in 116,069 procedures) for analysis of revision of any component. 79,192 procedures from Denmark, Norway, and Sweden were analyzed for the endpoint stem revision. Unadjusted survival rates were calculated according to Kaplan-Meier, and Cox proportional hazards models were fitted in order to calculate hazard ratios (HRs) for the risk of revision with 95% confidence intervals (CIs). Results — Unadjusted 10-year survival with the endpoint revision of any component for any reason was 92.1% (CI: 91.8–92.4). Unadjusted 10-year survival with the endpoint stem revision due to aseptic loosening varied between the stem brands investigated and ranged from 96.7% (CI: 94.4–99.0) to 99.9% (CI: 99.6–100). Of the stem brands with the best survival, stems with and without HA coating were found. The presence of HA coating was not associated with statistically significant effects on the adjusted risk of stem revision due to aseptic loosening, with an HR of 0.8 (CI: 0.5–1.3; p = 0.4). The adjusted risk of revision due to infection was similar in the groups of THAs using HA-coated and non-HA-coated stems, with an HR of 0.9 (CI: 0.8–1.1; p = 0.6) for the presence of HA coating. The commonly used Bimetric stem (n = 25,329) was available both with and without HA coating, and the adjusted risk of stem revision due to aseptic loosening was similar for the 2 variants, with an HR of 0.9 (CI: 0.5–1.4; p = 0.5) for the HA-coated Bimetric stem. Interpretation — Uncemented HA-coated stems had similar results to those of uncemented stems with porous coating or rough sand-blasted stems. The use of HA coating on stems available both with and without this surface treatment had no clinically relevant effect on their outcome, and we thus question whether HA coating adds any value to well-functioning stem designs.

Low revision rate after total hip arthroplasty in patients with pediatric hip diseases

Background The results of primary total hip arthroplasties (THAs) after pediatric hip diseases such as developmental dysplasia of the hip (DDH), slipped capital femoral epiphysis (SCFE), or Perthes’ disease have been reported to be inferior to the results after primary osteoarthritis of the hip (OA). Materials and methods We compared the survival of primary THAs performed during the period 1995–2009 due to previous DDH, SCFE, Perthes’ disease, or primary OA, using merged individual-based data from the Danish, Norwegian, and Swedish arthroplasty registers, called the Nordic Arthroplasty Register Association (NARA). Cox multiple regression, with adjustment for age, sex, and type of fixation of the prosthesis was used to calculate the survival of the prostheses and the relative revision risks. Results 370,630 primary THAs were reported to these national registers for 1995–2009. Of these, 14,403 THAs (3.9%) were operated due to pediatric hip diseases (3.1% for Denmark, 8.8% for Norway, and 1.9% for Sweden) and 288,435 THAs (77.8%) were operated due to OA. Unadjusted 10-year Kaplan-Meier survival of THAs after pediatric hip diseases (94.7% survival) was inferior to that after OA (96.6% survival). Consequently, an increased risk of revision for hips with a previous pediatric hip disease was seen (risk ratio (RR) 1.4, 95% CI: 1.3–1.5). However, after adjustment for differences in sex and age of the patients, and in fixation of the prostheses, no difference in survival was found (93.6% after pediatric hip diseases and 93.8% after OA) (RR 1.0, CI: 1.0–1.1). Nevertheless, during the first 6 postoperative months more revisions were reported for THAs secondary to pediatric hip diseases (RR 1.2, CI: 1.0–1.5), mainly due to there being more revisions for dislocations (RR 1.8, CI: 1.4–2.3). Comparison between the different diagnosis groups showed that the overall risk of revision after DDH was higher than after OA (RR 1.1, CI: 1.0–1.2), whereas the combined group Perthes’ disease/SCFE did not have a significantly different risk of revision to that of OA (RR 0.9, CI: 0.7–1.0), but had a lower risk than after DDH (RR 0.8, CI: 0.7–1.0). Interpretation After adjustment for differences in age, sex, and type of fixation of the prosthesis, no difference in risk of revision was found for primary THAs performed due to pediatric hip diseases and those performed due to primary OA.