Bone loss or poor bone quality is a major concern for any orthopaedic surgeon performing revision TKAs. As an increasingly younger patient population undergoes TKA, the orthopaedic surgery community must consider techniques and implants that may preserve bone for a potential revision TKA. We sought to evaluate the BMD about a new uncemented tibial implant with a lower modulus of elasticity to determine if this implant might limit the stress shielding seen with more traditional tibial baseplates.
There are some limitations to this study. First, the majority of the participants were male, which, although characteristic of a Veterans Administration Medical Center patient population, is not representative of other TKA populations. However, this controlled population did allow us to better control for variations in BMD based on age and gender. Second, no statements can be made regarding BMD changes beneath the TM tibial baseplate beyond 2 years. As survival of most TKA designs is excellent for the first few years after TKA, a longer-term evaluation might prove valuable. At least one longer-term followup study has shown BMD loss of as much as 36.4% at 8 years after TKA [14
]. Third, BMD changes after TKA may be most pronounced (and therefore most clinically relevant) in the distal femur [1
], but we did not examine this, as no tantalum ingrowth primary femoral component was available.
Accurately measuring and comparing bone density is challenging given the many variables that may affect bone density measurement and comparison. Osteoarthritis may alter the preoperative bone density of the operative and nonoperative knees, and pain secondary to osteoarthritis may limit the patient’s weightbearing, likewise affecting the bone density of either limb. Additionally, changes in anatomy and the weightbearing axis of the affected knee may increase density of the medial or lateral compartment in varus or valgus knees, respectively. Postoperatively, bone density may be affected similarly by weightbearing activity of patients and alterations in the mechanical axis of the limb. Finally, a potentially useful comparison to different TKA designs in the same patient was impractical as the sample size was small and the different implant design in the contralateral limb made zone modeling difficult. We acknowledge these limitations but believe the study population was too small to analyze multiple variables. Considering the logistics, patient commitment, and expense involved in this study of 41 patients (one of the larger single-design BMD studies), larger studies may prove impractical.
Quantifying BMD can be accomplished by numerous techniques such as ultrasound, quantitative CT, and DEXA. DEXA has the benefit of decreased radiation compared with CT. Also, recent software advances with DEXA technology allows for accurate measurement around arthroplasty implants. The software we used in this study to measure BMD can subtract the region occupied by the implant, measuring only the bone in question. The density is reported for the area as mg/cm2, not as total density, effectively negating the area occupied by the implant pegs.
Because of the unique design characteristics of low modulus and excellent bone ingrowth, it has been suggested a monoblock TM tibial component may be associated with a lower rate of osteolysis and component loosening, thereby leading to improved implant survivorship. A recent midterm study of this implant showed stable components with no evidence of osteolysis or loosening in 125 arthroplasties at a minimum 5-year followup [19
]. Additionally, a recent radiostereophotogrammetric analysis of this implant compared with a cemented tibial component showed cessation of subsidence after 3 months in the TM component, suggesting improved stability compared with previous metal-backed cementless designs [8
Previous studies examining BMD surrounding TKA tibial components have shown variable changes, although comparisons are limited by varying followups and implant designs. Seitz et al. [25
] reported a small decrease in uncemented tibial BMD 1 month postoperatively with a stemmed tibial component, with density returning to the immediate postsurgery level by 1 year. Bohr and Lund [4
] reported a 15% decrease in BMD 6 months after surgery, which returned to 90% of the immediate postoperative level by 3.5 years after the operation. Levitz et al. [14
] reported on seven patients followed for 8 years and observed inconsistent declines in tibial BMD in the first 6 weeks to 6 months after TKA, a return to the immediate postoperative density by 1 year, and a mean 36.4% decrease in BMD of the proximal tibia 8 years after TKA. In a more recent study reviewing BMD beneath an uncemented tibial implant, Petersen found BMD decreased 22% at 3 years [20
]. His study proposed alteration of the mechanical axis to a prearthrosis alignment may play a role in the decreased BMD noted postoperatively [20
]. Unlike these previous studies, we found the BMD unchanged from the preoperative measurement in two of the three ROIs examined. We believe the unique properties of the TM tibial implant may be responsible for maintaining the BMD in Zones 1 and 2. The material characteristics of tantalum closely match that of bone (cancellous 10–1723 MPa; cortical 12,000–18,000 MPa) with a modulus of elasticity of 5200 MPa [9
]. With biomaterial properties similar to that of host bone, stress shielding is minimized and load transfer across the implant should be more uniform. This may result in an increase in the loading of the bone in contact with the whole implant and minimize the stress shielding seen about the tibial implant that was reported in other studies [5
Some studies have sought to determine the role design of the tibial baseplate (stemmed versus pegged) or method of fixation (cemented versus ingrowth) might play in the changes seen in BMD. Lonner et al. [15
] and Bourne and Finlay [5
] found centrally stemmed tibial components had a greater impact on stress shielding of the proximal tibia than those without stems or with short pegs. In another study, changes in BMD beneath cemented and uncemented stemmed baseplates were compared. The authors found no difference in BMD between the two methods of fixation [1
The ROIs in our study were designed specifically for the implant in question. Zones 1, 2, and 3 were chosen because they might sustain stress shielding that could be clinically important with time. We believed Zone 3 was an especially important area as it represents all the bone immediately adjacent to and supporting the implant. The BMD in Zone 3 was seen to decrease substantially at 2 months in the operative knee. This is an interesting finding as Zone 3 includes Zone 1, with only the addition of small regions of bone outside the pegs, and Zone 1 did not have a similar decrease in BMD in the operative knee. As Zone 3 would include the more sclerotic medial/lateral plateau region characteristic of varus/valgus gonarthrosis, perhaps BMD decreases enough with the postsurgical removal of this bone to impact the ROI. Although the percent change in Zone 3 BMD from preoperatively to 2 months postoperatively differed between the operative and nonoperative knees, BMD did not increase with time in the nonoperative knee in this ROI. Possible explanations for this finding are diminished BMD with age or improved bilateral weight transfer after successful TKA. We also examined each ROI with time for each patient. Although the average BMD did not change in Zones 1 and 2 in the operative knee, we found the BMD increased in some patients and decreased in others. This is consistent with other studies of BMD of the proximal tibia after TKA showing broad ranges in BMD with large SDs. Abu-Rajab et al. [1
] reported SDs from 9.2% to 23.1% and Wang et al. [27
] reported SDs from 8.8% to 14.1% with BMD percent changes ranging from −30.9% to +36.5%. Our study showed SDs from 9.0% to 32.1% and BMD percent changes ranging from −68.8% to +100.3%. This shows the importance of analyzing percent change in BMD rather than an absolute value and reflects the number of variables that may impact proximal tibial BMD.
The potential for implants that prevent stress shielding and therefore potential bone loss has long-term implications for arthroplasty revisions. Bone loss attributable to stress shielding in periprosthetic bone has been described in multiple studies involving the distal femur and proximal tibia [1
]. Results of our study suggest BMD may be better preserved beneath a TM tibial baseplate compared with previous implant designs. Whether this will positively impact the anticipated TKA revision in the younger patient remains unanswered. Continued research of implants that minimize stress shielding and emphasize bone preservation is necessary as an increasingly younger patient population is considered for TKA.