Abstract: The Authors present a match-paired study between computer assisted and free-hand techniques using a short modular femoral stem in total hip arthroplasty (Metha, B.Braun Aesculap, Tuttelingen; Germany). They assessed surgical time, clinical outcome, dislocation rate, limb length and off-set in 44 patients with ideal indication for this more conservative implants. Despite both longer surgical time and similar early outcomes, the results demonstrated how computer assisted techniques permits an easier way to manage limb length discrepancy and off-set restoring. The Authors do believe navigated short modular stems as safe procedure towards a real less invasive surgery in hip arthroplasty.

Introduction: Tissue sparing joint replacement surgery has been a focus for many surgeons well before the recent enthusiasm for minimally invasive total hip replacement (THR) gripped the international orthopaedic community. However most of the attention given to this area has involved dedicated instruments to reduce the required surgical exposure. Likewise with a more frequent use in young and active patients bone saving procedures become more important and one of the goal is to save good bone stock for the revision procedure . Recently short stems preserving the femoral neck are available on the market to address these selected cases with first positive reports. Even more recently a new modular stem together with a navigation technology to support the selection of the right component and to optimize joint reconstruction has been proposed. Intraoperatively with the navigation the surgeon can evaluate limb length, medialization of the center of rotation and ROM.
Discrepancy of leg is often considered as a significant problem after Total Hip Replacement and has been associated to patient dissatisfaction. Pain, instability, stiffness, nerve traction and heteretopic ossification are described as direct and indirect consequences of a leg length discrepancy as well as of an incorrect off-set.
In literature different studies demonstrated substantial statistical improvement in accuracy of cup placement using navigation compared with freehand methods but very few studies regarding stem navigation are reported in literature and none evaluating the effect of navigation upon leg length discrepancy.
The Authors performed a matched paired study between 2 groups of modular short stem in hip arthroplasty: with (Ca-THR) o without the navigation support. They hypothesized that Ca-THR permits to achieve a better joint reconstruction with an effective control over the leg length discrepancy. Furthermore they compared the 2 groups according to hip function and number of post operative dislocations.

Materials and Methods: Twentytwo patients who underwent to a Ca-THR using using a CT-free computer assisted alignment system (Orthopilot 3.1, Aesculap, Tuttelingen; Germany) from April 2006 to January 2008 were included in the study (group A). All patients had a body mass index lower than 35. Patients with a displastic hip, limb length discrepancy bigger than 2 cm and with femoral neck /head major deformity were not included in this study because not ideal candidates for this implant. Every single patients in group A was matched with a patient who had undergone to a conventional free-hand THR (group B) between April 2006 and January 2008 in our hospital. Criteria of matching were age (with a maximum difference of + 3 years), sex, arthritis level, preoperative diagnosis and pre-operative limb length discrepancy (with a maximum difference of + 0.3cm). All the involved limbs were shorter or equal to the opposite limbs.
In both the group the same postero-lateral approach was used to implant the same prostheses (Metha modular short stem and Plasma-Cup, B.Braun Aesculap, Tuttelingen; Germany). Early weight bearing as tolerated was encouraged in all patients. The duration of surgery was documented in all cases.
Pre and postoperatively both limb discrepancy and the off-set were assessed radiologically using the method of Woolson et al. with IMPAX digital radiography software (Agfa-Gevaert, NV, USA). At the latest follow-up the off-set was calculated as the difference between the pre and post operative off-set values. All the radiographs were always taken with a standardized protocol with the same magnification. We have painstakingly educated and communicated with our radiographers to repeated if any mistake was detected. The radiographs were assessed by an independent radiologist blinded to the original procedure.
Furthermore at a minimum follow-up of 3 months the clinical outcome was evaluated using the Harris Hip Score and any dislocation was registered. Statistical Analysis was carried out using SPSS for Windows Release 11.0 (SPSS Inc, Chicago, Ill, USA). Data were represented as a mean and standard deviation for continuous response variables and as percentages for discrete variables.
Differences between the two groups were measured with an independent Student’s T test or Mann- Whitney non-parametric test depending on the data distribution of the continuous variables.

Results: No statistical differences in patients demographic data were registered. The mean followup was 10.8 months and 11.6 months respectively for group A e group B with no statistical
significant difference.
There were no significant differences in pre-operative limb length discrepancy between the 2 groups (mean 0.9 cm and 1.1 cm respectively for group A and B) (Tab 1).
In both the groups pre-operatively there were primary hyperthophic osteoarthritis in 18 patients, vascular necrosis in 3 patients and post-traumatic osteoarthritis in one (Tab 1).
No intraoperative technical problem was registered in both the groups.
In group A in 20 case a 32 mm ceramic head was used while a 28 mm ceramic head was used in 2 cases. In Group B a 32 mm head was used in 19 cases while a 28 mm ceramic head was used in 3 cases. In group A we registered a major variability in the femoral neck adapters (both in inclination and ante/retroversion) and neck size of the head (Fig1,2). The surgical time was statistically longer in group A with a mean of 102.6 minutes compared to 87.7 minutes in group B (Tab 2). At the latest follow-up no sign of major subsidence were present in all the implants. The mean discrepancy was statistically better reduced in the computer assisted group. In this group the mean post-operative discrepancy was reduced to 0.4 cm with no cases of discrepancy greater of 1 cm. In group B the mean post-operative discrepancy was reduced to 0.8 cm but with 2 (9 %) cases of discrepancy still greater of 1.0 cm. Postoperatively in both the groups no cases of leg discrepancy greater than 2cm was registered even if in 3 (13.6%) case in group B the discrepancy was increase of a mean values of 0.4 cm. The offset was better recreated in the computer assisted group compared to the freehand group with a statistical significant lower difference between the pre-op and post-op values (Tab 2).
There were no statistically significant differences in the Harris Hip score and all the patients were satisfied with the outcome. In group A the mean HHS score was 90.1 and 89 in gruop B futhermore we realized that the final outcome was still improving considering the short follow-up in some cases (Tab 2).
In the group B 1 patient experienced a traumatic dislocation because a car accident 7 months postoperatively with further 2 more recent atraumatic dislocations with no sign of implant loosening and he is scheduled for a THR revision. No cases of dislocation was registered in group A.

Discussion: Short stem prostheses represent an attractive alternative to resurfacing hip arthroplasty in the same selected cases. In combination with minimal invasive techniques permits to preserve muscular structure and bone stock avoiding complications related to resurfacing implants. The neck is partially maintained and the greater trochanter region remains untouched as well as the methaphysis is not filled leaving spongious bone.
Furthermore the newest implants associate short stem to modularity trying even to better restore the hip anatomy and biomechanics with no risks of mechanical failure.
Our main concern on this implants is the leg length discrepancy as well as in resufarcing implants. Lazovic in 2006 has already reported a significant percentage of elongation from 1 cm to 1.5 cm with the same implant even with a navigation support. Thus we do not use this implant routinately in “longer hips”.
Likewise different studies has already demonstrated better implant placement using navigation in THR either for the cup insertion or for the stem.
Navigation of short stem implants is mainly based on the restoration of the hip anatomy with no influence on the stem positioning. The navigation can evaluate intraoperatively the best modular neck and head size to achieve the planned off-set, leg-length and range of motion and even in our study in the navigated group we better exploited the different modular neck options.
At a minimum follow-up of 3 months after the surgical intervention, we performed a matched paired study comparing computer assisted THR to traditional free-hand THR using the same short modular stem. Our study has some limitations: it was retrospective and not randomised, with a too short follow-up to pick up any objective clinical difference between the 2 groups. Furthermore the series was too small even to demonstrate any reduction in the dislocation rate compared to the percentage reported in literature. However we tried to identify strict inclusion criteria (diagnosis, age and sex, body-mass index, shortening) to matched the best indications for this implant. We include patient with no elongation on the affected side and we tried to correct the discrepancy with this implant. No cases of primary atraumatic dislocation were registered in both the groups even in case with a longer follow-up.
Our results demonstrated how navigation in modular short stem can achieve a statistically significant better results both in correcting limb length discrepancy and in restoring the original off-set. In corrected indications the Authors do believe that navigated short femoral can represent a real less invasive procedure to restore a normal joint biomechanics with results at least similar to other conservative procedures but with lower complications.

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