Periprosthetic bone remodeling is complex and poorly defined, particularly with relation to the different cemented or uncemented prostheses currently in use. In our study, the comparative results revealed better preservation of periprosthetic bone with the uncemented HA-coated Furlong® prosthesis in Gruen Zones 2, 3, 5, and 6 on the femoral side and DeLee and Charnley Zone 1 on the acetabular side when compared with the cemented Charnley prosthesis.
Our study has several limitations. It lacks the baseline preoperative, postoperative BMD, and serial postoperative measurements of bone density; these data would have helped to identify longitudinal long-term changes resulting from the implantation and subsequent remodeling and also helped us to determine if the prosthesis has better preserved or increased the BMD in the periprosthetic region. Our primary aim was to compare the long-term difference in periprosthetic bone density between the cemented and uncemented HA-coated implants and the available data helped us to make that comparison. The followup of the patients who underwent cemented hip arthroplasties is approximately 4 years longer than for the patients who had the uncemented arthroplasties. Theoretically, the cemented hip prostheses have been in place longer and therefore, it might be expected that more bone was lost. Some studies have shown remodeling in the periprosthetic region varies with time and reaches homeostasis by approximately 2 years [1
] and few changes are expected later. Our study involves observing long-term homeostasis in bone remodeling and we believe the difference in timing of implantation of cemented and uncemented implants would not skew our results. Age has been found to negatively correlate with bone densities in femoral Zones 6 and 7 and acetabular Zones 2 and 3, suggesting older age is associated with lower bone densities. The small sample size in our study gives little power to detect such relationships. All radiologic observations were made once by one observer (PC), which could introduce interobserver and intraobserver bias and adds to the limitation of the radiologic findings.
There are no long-term studies that directly compare cemented and uncemented periprosthetic bone remodeling. In the short term, up to 4 years [12
], studies show that on the femoral side, the cemented prosthesis induced bone mass reduction in the lesser trochanter area and in the distal lateral cortex and with the uncemented stem produced significant bone resorption in the area of the lesser trochanter and in the medial and the lateral distal cortex of the femur; both induced an increase in the bone mass in the greater trochanter area. In comparison, our study showed a different pattern of bone remodeling with greater preservation of bone density in the distal medial and the distal lateral cortex with the uncemented stem. Longitudinal noncomparison studies with Charnley cemented prostheses [5
] have observed that at 1 year, there was a reduction in BMD of 6.7% in the calcar region and an increase of 5.3% in the femoral shaft distal to the tip of the implant [22
]. Studies with an uncemented HA-coated prosthesis [19
] showed that the BMD was lower in Gruen Zones 1 and 7 and concluded prosthesis design influences periprosthetic bone loss. Similar studies with HA-coated ABG prostheses showed bone density values averaged between 96% and 113.8% for Gruen Zones 1 to 6, and in Zone 7, there was a decline to an average of 72.1% at 24 months [24
]. In contrast, one study [17
] showed a 40% loss of bone proximally and 49% distally at 7 to 14 years [10
]. In our study with the HA-coated Furlong®
prosthesis, we observe preservation in the BMD in Gruen Zones 2, 3, 5, and 6 of the proximal femur. The difference in periprosthetic bone density observed in our study could be the result of the mechanical properties and load transmission characteristics of the prosthesis [25
]. The femoral stems in the Furlong®
prosthesis transmit load largely in the metaphyseal-diaphyseal region as compared with the Charnley prosthesis, suggesting a stress-related remodeling pattern for long-surviving prostheses.
On the acetabular side, the uncemented socket induced bone resorption at the medial and caudal zones, and the cemented socket showed significantly increased BMD in the cranial zone [12
]. Other studies with the cemented acetabular component observed the periacetabular bone mass returned to baseline values at 2 years with a pattern suggesting uniform transmission of load to the acetabulum [5
]. Our study showed the BMD was better preserved in DeLee and Charnley Zone 1 of the acetabulum. These observations reflect the difference in load transmission of the different socket designs suggesting a more uniform load distribution with the Charnley cups as compared with the screw-in design of the uncemented Furlong®
cups, which probably had a higher load transmission in Zone 1 [23
HA coatings induce bony ingrowth onto the surface of the prosthesis; its influence on the periprosthetic BMD is not clear. Although a fully HA-coated stem is expected to induce greater bone apposition, wider trabecular struts, and more connectivity compared with half HA-coated stems [2
], or tapered corundum-blasted titanium stems [1
], it is not clear if the HA coating has any role in better preserving bone stock, with studies showing good bony integration with uncemented nonbioactive surface implants [7
With good survival of the HA-coated Furlong®
cup and stem [22
], better preservation of periprosthetic bone would potentially make revision surgery less complex and also possibly decrease the risk of periprosthetic fracture. Additional long-term studies of remodeling and periprosthetic bone density between cemented and uncemented implants can help us understand preservation and behavior of periprosthetic bone stock.
Bone density is better preserved around the uncemented HA-coated stem in Gruen Zones 2, 3, 5, and 6 on the femoral side and Zone 1 on the acetabular side compared with the Charnley cemented stem.