The importance of reaming the posterior femoral cortex before inserting lengthening nails and calculation of the amount of reaming
© Kucukkaya et al. 2016
Received: 1 September 2015
Accepted: 11 January 2016
Published: 16 January 2016
Lengthening nails have been used to correct limb length discrepancy caused by different etiologies, as well as for post-traumatic reasons. Two important lengthening nail-related complications are damage to the distraction mechanism and femoral fractures around the nail tip. As a result of the curved anatomy of the femur, straight nails impinge on the anterior cortex. Therefore, proper reshaping of the medullary canal to accommodate straight lengthening nails is crucial for the prevention of this problem. Reaming the dense posterior cortex is important when aiming to insert a lengthening nail without incurring anterior cortex nail tip impingement-related complications. Posterior femoral cortex over-reaming is a solution to this situation.
Sixty patients received lengthening nails during 2008–2013, (ISKD, Fitbone, Precice). Posterior cortex rigid-reaming technique was used successfully in 45 retrograde femoral lengthening cases. The preoperatively planned posterior cortex amount was reamed until the impingement was overcome during the operation under fluoroscopic control for each case. Since the preoperative determination of posterior cortex reaming amount is time consuming and operator dependent, we evaluated the X-rays of the patients with computer software and conventional paper-based measurements. The effect of reaming the posterior cortical wall on the inclination of the nail tip to the anterior femoral cortex was detected with measurements on the preoperative and postoperative lateral femoral X-rays by using the CorelDRAW® Graphic Suite X6 software package (Corel, Inc., Ottawa, Ontario, Canada) software. On the same software, X-rays and the posterior reaming amount were also calculated.
The mean age of the patients was 27 years (11–42), while the mean lengthening was 5.9 cm (2–14). The mean consolidation index was 1.05 (0.75–1.62), and the mean follow-up period was 31 months (range, 18–45 months). The mean distance of the osteotomy site to the intercondylar notch of the femur was 81.2 mm (±16.92). The mean displacement of the nail tip position was 15.42 mm (±4.77) on the measurements on the postoperative X-rays after nail insertion compared to the preoperative simulations on the templates. The mean posterior cortex reaming thickness was 3.68 mm (±1.02).
We derived a formula that allows the required amount of optimal posterior cortex reaming to be determined. No impingement-related complications or nail damage were observed.
Treatment techniques for leg-length discrepancies and deformities caused by post-traumatic or other reasons have developed substantially over the past decade. These techniques have been improved to allow the application of fully implantable lengthening nails [1, 2]. Lengthening nails are now available with various distraction mechanisms, such as mechanical (the Intramedullary Skeletal Kinetic Distractor (ISKD); Orthofix, McKinney, TX, USA), motor-driven (Fitbone nail; Wittenstein Intens, Igersheim, Germany), and magnetic (PRECICE; Ellipse Technologies, Irvine, CA, USA). Regardless of the lengthening mechanism, these devices are either straight or have an acute proximal angle. The main objective of this study was to emphasize the importance of rigid posterior femoral cortex reaming to avoid complications when inserting straight lengthening nails.
We therefore (1) evaluated the effect of reaming the posterior cortical wall on the inclination of the nail tip to the anterior femoral cortex and (2) determined the optimal amount of posterior cortical reaming required with a rigid reamer using the CorelDRAW® Graphic Suite X6 software package (Corel, Inc., Ottawa, Ontario, Canada) and conventional paper-based measurements.
Fifty-eight patients with 65 femoral shortenings of various etiologies underwent surgery with lengthening nails (ISKD, Fitbone, Precice). The etiologies of the femoral shortenings were malunion in 17 cases, physeal arrest in 12, poliomyelitis in 7, congenital hypoplasia in 8, achondroplasia in 1, and Legg-Calve-Perthes disease sequela in 2. There were five cases of idiopathic femoral shortenings and six cases underwent the procedure for cosmetic reasons. The cosmetic cases, and one of the achondroplasia cases, were treated for bilateral femurs. The retrograde insertion and rigid reaming techniques were used in 45 femora, and the antegrade insertion technique was used in 20 femora. The mean age of the patients was 27 years (range, 11–42 years), while the mean lengthening was 5.9 cm (range, 2–14 cm). The mean consolidation index was 1.05 (0.75–1.62), and the mean follow-up period was 31 months (range, 18–45 months). After the consolidation phases were complete, the patients were followed up at 3-month intervals during the first year. After the first year of consolidation, the follow-up was conducted yearly.
To detect the effect of reaming the posterior cortical wall on the inclination of the nail tip to the anterior femoral cortex in our cases, the preoperative and postoperative lateral femoral X-rays of the patients were uploaded to the Coral draw software. The template of the lengthening nail to be inserted was placed onto the preoperative X-rays without posterior cortex reaming so that the position of the nail tip without posterior cortex reaming and osteotomy was simulated. The distance of the nail tip to the posterior cortex was measured. Then, the distance of the nail tip to the posterior cortex was measured on the postoperative lateral femoral X-rays on which the nail tip was located in the intramedullary canal after posterior cortex reaming and angulation at the osteotomy site. By using these measurements, the displacement of the nail tip was calculated. The thickness of the thickest part of the posterior femoral cortex was measured on the preoperative and postoperative lateral femoral X-rays, and the posterior reaming amount was calculated.
Calculation of the amount of posterior cortex reaming required—“Software and conventional paper method”: sample case
We then determined the amount of posterior cortical reaming required with a rigid reamer using the CorelDRAW® Graphic Suite X6 software (Corel, Inc.) and the conventional paper-based technique and derived a formula to allow the surgeon to predict the optimum amount of posterior cortex reaming required.
This study has been approved by “Istanbul Bilim University Clinical Researches Ethical Board” with the reference number 44140529/2014, and it is compliant with the Helsinki Declaration. An informed consent has been received from all participants or legal guardians for participation to the study.
The mean distance of the osteotomy site to the intercondylar notch of the femur was 81.2 mm (±16.92). The mean displacement of the nail tip position was 15.42 mm (±4.77) on the measurements on the postoperative X-rays after nail insertion compared to the preoperative simulations on the templates. The mean posterior cortex reaming thickness was 3.68 mm (±1.02).
Sample case results
Because the femur is bow-shaped in the sagittal plane, there are difficulties in adapting straight nails to the femoral anatomy. In femoral lengthening cases with both retrograde and antegrade lengthening nails, anterior nail tip impingement is a major problem that can cause femoral fractures or nail dysfunction. A straight lengthening nail can be inserted after reasonable over-reaming with a flexible reamer. However, posterior cortex rigid reaming, which solves the anterior nail tip impingement issue, is inevitable in cases with a narrow medullary canal or in patients who have an overcurved femur. The formula derived from our study may help the surgeon to easily predict the required amount of posterior cortex reaming. We experienced no anterior cortex impingement-related complications in our 65 femoral lengthening cases.
The anterior incline of the nail tip can be affected by sagittal bowing of the medullary canal or osteotomy level. The anterior incline of the nail tip decreases as the osteotomy level approximates the apex of the bow of the medullary canal. Buford et al.  built a cadaveric femoral model based on three-dimensional computed tomography. They calculated the mean sagittal radius of curvature (ROC) of the femur as 144.6 ± 39.7 cm. They also identified no difference in the ROCs between the anterior cortex and the medulla. Their study did not report the amount of posterior cortical bowing. However, the femur is more bowed in the posterior wall than in the anterior wall and medullary canal because the posterior femoral cortex has a greater thickness. Kanawati et al.  compared two intramedullary nails with ROCs of 150 and 200 cm after placement into synthetic femora. They reported that the amount of nail tip impingement on the anterior femoral cortex of the second nail was higher and recommended using nails with low ROCs to avoid anterior cortical impingement during applications. The lengthening nails are straight because at present the technology does not allow for the production of curved lengthening nails. Therefore, to produce a smooth insertion, it is crucial to adapt the shape of the femur to that of the straight nail.
The major limitation of this study was that we developed our formula using a healthy femur and a standard-sized nail. Therefore, the application of the formula to a femur with variations such as increased bowing or any other deformity in the coronal plane could cause technical difficulties such that the formula may need to be adjusted for each femoral variation.
In conclusion, our formula is reliable in predicting the optimum amount of posterior cortex rigid reaming in femoral lengthening cases. The calculation of the amount of posterior cortex reaming during preoperative planning may not be necessary in all cases. However, preoperative planning should be carried out in all cases, especially in cases involving a narrow medullary canal, or patients with an overcurved femur. In addition, each patient should be evaluated separately in terms of any structural and histological variations. Our formula can be used as a mathematical proof of the planning. Our study can also open the way for developing more specific calculations for different clinical situations.
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