67Bulletin • Hospital for Joint Diseases Volume 60, Number 2 2001-2002

AbstractRecently, a new, shorter IM nail using two 6 mm recon-struction screws for proximal fixation was introduced intwo versions for femoral insertion: piriformis fossa(FAN) and greater trochanter (TAN). These nails werecompared experimentally for their fixation stability,proximal load transmission, and failure strength in anunstable intertrochanteric fracture model in cadavericfemurs. Vertical and axial loads were first applied to theintact femurs. Fractures were created, subsequent fixa-tion applied, and the femurs underwent a series of bothvertical and axial loading tests. There was no signifi-cant difference in strain readings between the nails foreither axial loading or cyclical loading. There was nostatistically significant difference between the loads tofailure for the trochanteric nails and the standardantegrade nails. The average ultimate load for the FANand TAN nails were 3010 N and 2830 N respectively.These two nails performed very similarly throughout ourtesting

Standard intramedullary (IM) nails are usually usedfor subtrochanteric and more distal femoralfractures; shorter IM nails such as the Gamma

(Stryker, Howmedica-Osteonics, Rutherford, NJ), andIMHS (Smith & Nephew, Memphis, TN) are used for moreproximal fractures. The former usually rely on two 6 mmto 7 mm proximal fixation screws in the femoral head and

neck; the latter, on a large single screw similar to the slidinghip screw.

One of the problems with some of the longer IM nailsis the insertion point in the proximal femur.1 The piri-formis fossa is difficult to get to, and often the fracturemay involve this area, so that insertion of the nail at thispoint results in further comminution of the fracture. Pre-vious studies have also shown an increased risk of femo-ral fracture during the insertion of the nails, due to thefact that the femur is bowed while the nails are straight.To avoid these problems, however, long Gamma (Stryker,Howmedica-Osteonics, Rutherford, NJ) and IMHS(Smith & Nephew, Memphis, TN) nails are available.

For the shorter nails, there are two types of proximalscrew configuration available. For nails inserted into thepiriformis fossa, two proximal locking screws are usedfor fixation of the head and neck. For nails inserted intothe greater trochanter, one large lag screw is used forfixation in the head and neck. When comparing thesetwo types of nails, it is unclear whether the insertionpoint or the type of proximal fixation is primarily re-sponsible for any differences.

Recently, a new, shorter IM nail using two 6 mm re-construction screws for proximal fixation was introducedin two versions for femoral insertion: piriformis fossa(FAN) and greater trochanter (TAN) (Fig. 1). Becausethese two IM nails differ in design (the TAN has a proxi-mal bend), they have differences in their accommoda-tion in the medullary canal and slight differences in fixa-tion screw lengths (Fig. 2). Both nails use two proximalreconstruction screws at 135° for proximal fixation, sothe only significant design difference is the bend allow-ing for trochanteric insertion. Thus, unlike the previouslymentioned nails, theses nails differ by only one variable,rather than two. The following experiment comparedthem with respect to fixation stability, proximal load

Effect of Piriformis Versus Trochanteric StartingPoint on Fixation Stability of Short IntramedullaryReconstruction Nails

Edward T. Su MD Hargovind DeWal MD Roy Sanders MD Frederick J. Kummer PhDMohammed Mujtaba MD and Kenneth J. Koval MD

Edward T. Su, M.D., Hargovind DeWal, M.D., Frederick J.Kummer, Ph.D., Mohammed Mujtaba, M.D., Kenneth J. Koval,M.D., are in the NYU-Hospital for Joint Diseases Department ofOrthopaedic Surgery, New York, New York. Roy Sanders, M.D.,practices orthopaedic surgery in Tampa, Florida.Reprint requests: Edward T. Su, M.D., NYU-Hospital for JointDiseases Department of Orthopaedic Surgery, 301 East 17thStreet, New York, New York 10003.

68 Bulletin • Hospital for Joint Diseases Volume 60, Number 2 2001-2002

transmission, and failure strength in an unstable inter-trochanteric fracture model.

Methods and MaterialsTen matched pairs of osteopenic embalmed cadaver fe-murs were selected on the basis of a bone density of 0.3to 0.5 g/cm2 (QDR-2000 Supine Lateral X-Ray Bone

Densitometer, Hologic, Waltham, MA). Radiographs intwo planes were taken to exclude samples with morpho-logic abnormalities. The femurs were stripped of all softtissues and the distal femoral condyles were removed ata level 15 cm distal to the inferior edge of the lessertrochanter. The femoral shafts were then potted with alow melting lead-tin alloy in 6 cm x 20 cm steel tubes.The specimens were wrapped with saline soaked gauzeand sealed in airtight double bags when not in use toavoid desiccation throughout the experiment.

One linear strain gauge was mounted on the medialaspect of each femur using cyanoacrylate cement, onecentimeter inferior to the planned location of the frac-ture site. An electronic displacement gauge was attachedto the femur, just distal to the proposed fracture site,parallel to the axis of the femoral shaft, with the springgauge element contacting the inferior femoral head tomeasure inferior displacement of the head with axialloading. The specimens were secured on the testing plat-form with a vise at 25° adduction in the coronal plane,and neutral in the sagittal plane to simulate one-leggedstance.2 Mechanical testing of the intact femurs was per-formed to provide control values for femoral strain andinferior head deflection in axial loading. A MTS Mate-rial Testing Machine (MTS, Minneapolis, MN) was usedto apply vertical loads directly onto the femoral headwith a flat applicator to allow free horizontal movementof the femoral head. The axial load was applied in incre-ments of 250 N to 1000 N; the medial strain and theinferior head displacement was recorded after ten sec-onds of loading.

The nails used for this study were the Smith &Nephew TAN and FAN nails (Smith & Nephew, Mem-phis, TN). The holes for the proximal locking screws

Figure 1 Short intramedullary reconstruction nails. Note the TAN(left) has a 5° bend, and the FAN (right) is straight.

Figure 2 Radiographs of a pair of femursafter fixation with the TAN (left) and FAN(right). The lateral placement of the TAN re-sults in a slightly increased lever arm.

69Bulletin • Hospital for Joint Diseases Volume 60, Number 2 2001-2002

are angled at 135° in both nails. The only difference be-tween the nails is a 5° bend 8 cm from the top of theTAN nail to allow for trochanteric insertion. They bothhave a hole and slot distally for distal interlockingscrews.

One femur from each matched pair was randomly as-signed to the TAN group. The other femur was assignedto the FAN group. Using a thin-blade oscillating saw, afour-part unstable intertrochanteric hip fracture was cre-ated. The first fracture line was created through the in-tertrochanteric ridge. A second fracture line was createdaround the lesser trochanter. This fracture line was ex-tended obliquely and superiorly to the anterior portionof the intertrochanteric fracture line. Finally, a transversefracture line was created at the base of the greater tro-chanter.3 The fractures were reduced and the instrumen-tation was performed under direct vision and fluoro-scopic guidance. Starting holes were first drilled in eitherthe greater trochanter or piriformis fossa, as appropriatefor each nail (Fig. 3). The nails were then inserted withthe alignment jig. The 6.0 mm reconstruction screwswere placed, using the alignment jig, so that the inferiorscrew was resting on the medial calcar of the femoralneck. The superior screw was located in the center ofthe femoral head. Screw lengths were measured bothradiographically and directly with a depth gauge. Thescrews ended in the subchondral bone less than 1 cmfrom the articular surface. Distal interlocking screwswere not used in any of the femurs, as the alignment jigfor distal locking screws for the TAN was not availablefor the experiment. Also, distal interlocking screws arenot essential for stable fixation of this proximal fracturepattern.

After fracture fixation, the femurs were again loadedaxially, with 250 N increments to 1000 N, while record-ing the medial femoral strain and the inferior head dis-placement at each load interval. The femurs were thencyclically loaded to 750 N for 100, 1000, and 10,000cycles in a sinusoidal manner at 3 Hz. After each cycle

Figure 3 Placement of the nails. The FAN(left) is inserted through the piriformis fossa.The TAN (right) is inserted through a start-ing hole in the greater trochanter.

Figure 4 Loading to failure. Most of the femurs failed by exten-sion of the fracture under the lesser trochanter.

70 Bulletin • Hospital for Joint Diseases Volume 60, Number 2 2001-2002

interval, the inferior head displacement and strain wererecorded both with and without loading to 750 N.

Finally, the femurs were then loaded to failure. Axialloads were applied to the femoral head at a rate of 1.0cm/min, continuously recording load until failure, whichwas defined as fracture of the femoral head, neck, orshaft, extension of the prior fracture, screw cutoutthrough the head, or deformation of the implants.

Analysis of DataThe data for the FAN- and TAN-fixed matched pairs wereanalyzed using paired t-tests and repeated measuresanalysis of variance to assess differences between treat-ment techniques. A significance level of α = 0.05 wasused.

ResultsThere was no significant difference in strain readingsbetween the nails for either axial loading or cyclical load-ing. Due to variations in insertion technique, 4 femursfrom each group were found to have a 1 to 2 mm gap inthe medial cortex fracture after fixation. This gap pre-vented load transfer across the fracture, and hence therewere no significant changes in strain along the medialcortex. There was a significant difference in strain curvesand displacement curves when compared to the curvesof the intact femurs. The slope for the strain curvechanged from -0.560 to -0.166 after fixation with theFAN device (p < 0.05). The slope for the strain curvechanged from -0.605 to -0.325 after fixation with theTAN device (p < 0.05). The slope for the displacementcurve changed from 0.160 to 1.48 after fixation with theFAN device (p < 0.05). The slope for the displacementcurve changed from 0.168 to 1.75 after fixation with theTAN device (p < 0.05).

Axial LoadingAt a load of 1000 N, the average inferior displacementof the femoral head was 2.64 mm for the femurs fixedwith the TAN nail, and 2.09 mm for those fixed with theFAN nail (Table 1). The difference was not significantwhen analyzed with repeated measures analysis of vari-ance or with paired t-tests.

Cyclical LoadingNone of the specimens failed at 100, 1000, or 10,000cycles. There was no statistically significant differencein inferior head displacement after 100 or 1000 cycles.After 10,000 cycles, the mean inferior head displace-ment of the FAN-fixed femurs was 2.02 mm, whereasthe mean inferior head displacement of the TAN-fixedfemurs was 3.03. Although these differences were notsignificant by repeated measures analysis of variance,the values at 10,000 cycles were significantly different(p < 0.05) when analyzed by a paired t-test (power 60%).

Table 1

FAN TANNail Nail t-test

Strain 750 N(mmm/mm) -133 -214 p > 0.05Displacement 750 N (mm) 1.98 1.44 p > 0.05Strain 1000 N(mmm/mm) -176 -293 p > 0.05Displacement 1000 N (mm) 2.09 2.64 p > 0.05

Load to FailureThere was no statistically significant difference betweenthe loads to failure for the trochanteric nails and the stan-dard antegrade nails. The average ultimate load for theFAN and TAN nails were 3010 N and 2830 N respec-tively. Nine of the ten femurs from each group failed byextension of the fracture through the lesser trochanterarea (Fig. 4). The other femur from each group failed byhead fracture. These two femurs were from the samematched pair.

DiscussionDuring static loading, there was no significant differ-ence between the FAN and TAN nails. It is likely thatany differences in strain were too small to detect withthe sample size used in this study. The only significantdifference found between the two nails was a 1 mm dif-ference in the mean inferior head displacement after10,000 cycles. This may have been due to toggling ofthe TAN nail within the canal, as there is slightly morespace medially between the nail and the medial cortexwith the trochanteric insertion. This 1 mm difference maynot be clinically significant. We found no significantdifferences in the strain readings between the two nails.Several femurs were found to have an approximately 1mm to 2 mm gap in the medial cortex above the straingauge after fracture fixation. This probably more closelyapproximates the position of fracture fragments whenfixed in vivo, as the reduction of the medial cortex can-not be visualized intraoperatively to ensure anatomicreduction. This gap usually closed down with loading,but the strain readings did not significantly differ for theloads when the gap was closed. The TAN reconstructionscrews may be expected to be slightly longer due to thebend in the nail, but this difference was not noticed inthis study, perhaps because the screw lengths are pro-vided in increments of 5 mm. Since deflection is pro-portional to the length of the reconstruction screw, thismight be a source of difference in deflection and possi-bly medial strain, but the difference would be too smallto detect in this study.

This study had several limitations. Although the num-ber of femurs in each study group was large comparedto other studies, it was still small enough to limit thepower of the study. Despite this, the difference in headdisplacement after 10,000 cycles was statistically sig-

71Bulletin • Hospital for Joint Diseases Volume 60, Number 2 2001-2002

nificant, with 60% power. A four-part unstable intertro-chanteric fracture pattern was chosen for this study. Otherfracture patterns could have been used in this study, andthese might have yielded different results. However,some fracture patterns require an abductor force, whichis too technically difficult to reliably reproduce whencycling the load for 10,000 cycles. Also, the more un-stable fracture pattern would be more likely to revealdifferences in stability of fixation between the two nails.

The ultimate loads to failure for the FAN-fixed andTAN-fixed femurs were 3010 N and 2830 N, respectively,compared to the values found for three other intramed-ullary devices by Wheeler and colleagues.4 These val-ues were 3870 N for the Richards reconstruction nail,3110 N for the Zimmer reconstruction nail, and 2500 Nfor the Synthes spiral blade reconstruction nail. It maybe inappropriate, however, to compare these values, asthe study by Wheeler and colleagues used a differentfracture pattern, a different testing protocol, and a dif-ferent selection of femurs.

These two nails performed very similarly throughoutour testing. In this study, the entry point had no effecton fixation stability, proximal load transmission, or fail-ure strength. The 1 mm difference after 10,000 loadcycles is probably clinically insignificant. As both nailsused two proximal reconstruction screws, the entry por-tal appeared to be the primary difference between thegroups.

It is unclear how these nails will compare to othersecond-generation short intramedullary nails, such as the

Gamma (Stryker, Howmedica-Osteonics, Rutherford,NJ), and IMHS (Smith & Nephew, Memphis, TN). TheGamma has a 10° bend, and has been suspected of hav-ing a higher fracture rate due to three-point loading.5

The IMHS has a 4° bend, which allows it to align itselfmore anatomically in the intramedullary canal. The TANnails in this study have a 5° bend, which may be ex-pected to fit more like the IMHS, but the TAN has twoproximal reconstruction screws instead of the one largelag screw with a screw sleeve construct. This differencewas not evaluated in this study. Perhaps a future studycomparing the TAN and FAN nails with the Gamma andIMHS devices may be indicated.

References1. Ebraheim NA, Mekhail AO, Checroun AJ: Entry point of

reconstruction nail. Am J Orthop 27(6):474-476, 1998.2. Chang WS, Zuckerman JD, Kummer FJ, Frankel VH: Bio-

mechanical evaluation of anatomic reduction versus medialdisplacement osteotomy in unstable intertrochanteric frac-tures. Clin Orthop 225:141-146, 1987.

3. Walsh ME, Wilkinson R, Stother IG: Biomechanical stabil-ity of four-part intertrochanteric fractures in cadaver femursfixed with a sliding screw-plate. Injury 21(2):89-92, 1990.

4. Wheeler DL, Croy TJ, Woll TS, et al: Comparison of recon-struction nails for high subtrochanteric femur fracture fixa-tion. CORR 336:231-239, 1997.

5. Rantanen J, Aro HT: Intramedullary fixation of high subtro-chanteric femoral fractures: A study comparing two implantdesigns, the Gamma nail and the Intramedullary Hip Screw.J Orthop Trauma 12(4):249-252, 1998.