Early femoral stem subsidence reportedly predicts long-term failure [8
]. Subsidence greater than 1 mm within the first 2 years is considered a major concern, and when the mean subsidence in a population is between 1 and 2 mm, the risk of clinical failure after 5 to 10 years is estimated to be between 25% to 50% [9
]. Despite this, several well-functioning cementless femoral stems show some degree of early migration, predominantly in the form of subsidence and rotation to retroversion [18
], most likely owing to stem impaction associated with loading during weightbearing [20
The design of the stem and close apposition of implant and bone are primarily responsible for initial stability of the stem [6
]. The aim of treatment for intraoperative femoral fractures is to create an environment that closely matches the intact bone in terms of initial stability. The treatment method depends on the severity and location of the fracture and stability of the prosthesis. Minor cracks in conjunction with a stable implant often are managed with cerclage wiring or other minor technique modifications that achieve continued stability [1
]. However, when monitoring a patient’s outcome, the ability to detect instability attributable to early stem migration using conventional radiography is limited. Malchau et al. [12
] suggested stem migration is detectable only if it exceeds 5 to 7 mm when using bony landmarks and conventional radiography. Therefore, when evaluating stability after fracture repair during the early postoperative phase, the use of conventional radiography may underestimate the amount of instability occurring between the implant and bone. The use of RSA during postoperative evaluation of this patient enabled us to detect a large amount of initial subsidence, indicating early postoperative stem instability despite cerclage fixation, and provided an accurate measure of subsequent implant restabilization.
Our patient experienced marked subsidence and rotation to retroversion within the first 6 postoperative months. From the data we obtained, it is not possible to determine how soon after fracture fixation stabilization occurred, as there were no measurements made before 6 months. It also is not possible to determine the exact amount of total subsidence as any subsidence that occurred before the initial postoperative reference examination at 4 days is excluded. However, Strom et al. [19
] reported there is little micromotion within the first postoperative week and reference measurements taken within 1 week of surgery seem sufficient. Therefore, it is likely early stabilization was achieved, as reported for other cementless stems [19
]. Late stem subsidence related to injury was reported in one case during study of the effect of early weightbearing on stem migration [20
]. This patient experienced 2.7 mm subsidence and 12.8º retroversion after falling on the hip before a 2-year RSA examination. No fracture could be detected and the stem restabilized by 3 years [20
We acknowledge the short followup of 2 years. Although it is recognized early stem micromotion is an indicator of later loosening [8
], the results seen with our patient alone do not necessarily ensure stability will be maintained during a 5- to 10-year period. The 5-year RSA measurements for our patient will provide a more accurate prediction of long-term success. Another limitation of this study is the absence of a precision calculation. However, it is likely the precision will be similar to those reported in another study (between 68 μm and 138 μm depending on axis) [4
] owing to similarity in the spatial distribution of the markers on the stem and in the bone.
Instability after cerclage fixation has been seen in animal studies. A study of micromotion and bone ingrowth into porous-coated canine femoral stems showed a deleterious effect of fracture despite cerclage wiring, resulting in an increase in stem micromotion and a reduction in bone ingrowth in comparison to unfractured femurs [15
]. Micromotion at the bone implant interface is known to promote fibrous tissue formation, resulting in poor bone anchorage. Although stem design is known to affect initial stability, surface finish contributes substantially to osseointegration and long-term stability [6
Our case presented a unique opportunity to observe initial stem micromotion occurring after cerclage fixation of an intraoperative fracture during THA. The findings show, despite a large amount of initial subsidence and retroversion, implant stability was achieved by 6 months and subsequently maintained. This unique observation of a stabilizing pattern of migration after fracture emphasizes the importance of interpreting the pattern of migration using RSA after insertion of cementless components, where initial subsidence and retroversion may be attributed to mechanical compaction in the host bone. The migration pattern subsequent to an initial period of osseous integration is likely to be a more important predictor of long-term stability than the initial migration measurement alone.