As majority of the femoral stems for primary THA were designed as proximally coated[19], the changes of proximal periprosthetic BMD,namely Gruen zone 1 and 7 (Fig 1), were suggested to bemore clinically relevant than those of other Gruen zones. In consistent with our results, previous studyalso found that the decreasesof the mean BMD in Gruen zone 1 and 7 variedfrom 5% to 10% during the first two years after THA[7].As the mean changes of BMD inGruen zone 1 (−0.033 g/cm2), Gruen zone 7 (−0.057 g/cm2),and total Gruen zones (−0.025 g/cm2)were larger than the LSC (0.012 g/cm2), we believe that our results represent a real biological change[20].
As we mentioned before, not all the patients experienced periprosthetic bone loss after THA[7, 8], while earlymedical intervention was recommended[5, 6].However, no predictive tool that enablesquantified individualized risk evaluations of postoperative periprosthetic bone loss on bias of numerous variableswas available till now.Nomograms are a pictorial representation of a complex mathematical formula designed to allow the approximate graphical computation[21]. Points at respective horizontal axis in nomograms represented the predictive value of the variables. After calculation of the total risk score based on the patients’ response for each variable, surgeons could correlate it to a specific chance of having the given outcome. Recently, nomograms have been used widely in predicting clinical related outcomes after total joint arthroplasty, such as 30-day/90-day readmission[22, 23], major complications[24], and excess cost within bundled payment[25]. Those prediction modelsthatindividualized the predicted outcome to specific patients’ characteristics performed better than simply relying on the average outcome[18]. To the best of our knowledge, our study represented the first time to use the nomograms in estimating the risk of periprosthetic bone loss. In the present study, variables (age, BMI, implant design, et al.) that have been reported to be potential risk factors of postoperative periprosthetic BMD decreaseswere retrospectively collected to create the nomograms[9-14]. The concordance index in binary outcomes predicting models represents the ability to distinguish between patients who experience an event from those who do not. It is measured on a scale of 0.5 (no better than chance) to 1 (perfect discrimination)[21]. As the bias-corrected concordance index of Gruen zone 1, 7 and total ranged from 0.696-0.785 in the present study, we proposed that those nomograms estimating the risk of periprosthetic bone loss had moderate to strong discrimination[21].
In the present study, we found that the most highly influential factor for postoperative bone loss in Gruen zone 1, 7 and total zones was BMD in corresponding Gruen zones measured one week after THA. As we discussed in our previous study[14], the trabecular bone of proximal femur became granular shaped and was located mostly in the interface between the implant and host bone after implantation of the femoral prosthesis. Similar to autogenous cancellous bone grafting, the trabecular bone would be completely eliminated before the new bone formation, which we supposed to be a reasonable explanation[26]. Previous study demonstrated that younger patients have more postoperative daily living activities and corresponding accelerated periprosthetic bone remolding[27]. Similarly, we also found that age was negatively related to the postoperative bone loss in Gruen zone 1, 7, and total zones. In consistent with previous studies[13, 28], preoperative hip BMD was found to be predictable of less postoperative periprosthetic bone loss in the present study. Similar to our results, the meta-analysis reported that patients using straight stems experienced less bone loss than those using anatomic designs at the 1-year time point[12]. Nevertheless, further studies with larger scale and specific stem design groupings are necessary to determine its’ clinical relevance, as cementless anatomic stems were reported to be with satisfied survival rate at 10 years (>95%)[29].
Bisphosphonate is a class of anti-bone-resorptive agents including alendronate, risedronate, ibandronate, and zoledronic acid, et al. FDA-approved indications for bisphosphonates include treatment of osteoporosis in postmenopausal women, osteoporosis in men, glucocorticoid-induced osteoporosis, hypercalcemia of malignancy, Paget disease of the bone, and malignancies with metastasis to the bone[30]. Recently, several prospective studies demonstrated that the administration of bisphosphonate effectively inhibited postoperative periprosthetic bone loss from one to three years after the THA[13, 31-34]. However, there is no clear guideline regarding the indication of bisphosphonate treatment for patients underwent THA, especially for those without osteoporosis and osteopenia. Traditionally, clinicians used their individual or group evaluation of the risk of postoperative periprosthetic bone loss as the basis of making clinical decisions, which has been proven to be subject to biases[18]. A prediction model that allows estimation of postoperative periprosthetic BMD changes at perioperative period could enable efficient identification of patients who benefit more from bisphosphonate treatment and individualized decision-making. Such prediction model could also provide patients with reasonable expectations following surgery, which may improve satisfaction and patient compliance.
Our study also subjected to some limitations. Firstly, patientsenrolledin the present study were relatively young (63, (51, 67) years, presented as median (Q1, Q3)). External validation is needed before the application of those nomograms on older (>80 yrs.) or much younger patients (<40 years).Secondly, although the sample size of the present study has met the requirement of the statistics, we admitted that large-scale sample is needed for building nomograms with higher discrimination and calibration. Additionally, the present study included a relatively short follow-up period. Nevertheless, it was reported that the periprosthetic bone loss was most evident and clinically relevant in the first year after THA,as the initial periprosthetic bone remodeling process was mainly completed in the first 12 postoperative months[4, 35]. Lastly, although the data was collected from a high-volume joint center that has a complex patient population, selection bias still existed due to the retrospective, single-center design.