Retrograde Femoral Nailing in Elderly Patients: Outcome and ...

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Retrograde Femoral Nailing in Elderly Patients: Outcome and Functional Results Thomas Neubauer, MD; Manfred Krawany, MD; Lukas Leitner; Alois Karlbauer, MD; Michael Wagner, MD; Michael Plecko, MD

abstract Full article available online at Healio.com/Orthopedics. Search: 20120525-24 Functional outcome after retrograde femoral intramedullary nailing was investigated in 35 patients older than 60 years (mean, 86 years) with 36 fractures, comprising 15 (41.7%) shaft and 21 (58.3%) distal fractures; overall, 7 (19.4%) periprosthetic fractures occured. Twenty-two (62.9%) of 35 patients were evaluated at a mean 16.5-month follow-up with the Lyshom-Gillquist score and the SF-8 questionaire. Primary union rate was 97.8%, with no significant differences in duration of surgery, bone healing, mobilization, and weight bearing among different fracture types; periprosthetic fractures revealed a significantly delayed mobilization (P5.03). Complications occured significantly more often among distal femoral fractures (P5.009), including all revision surgeries. The most frequently encountered complication was loosening of distal locking bolts (n53). Lysholm score results were mainly influenced by age-related entities and revealed fair results in all fractures (mean in the femoral shaft fracture group, 78.1 vs mean in the distal femoral fracture group, 74.9; P5.69), except in the periprosthetic subgroup, which had good results (mean, 84.8; P5.23). This group also had increased physical parameters according to SF-8 score (P5.026). No correlation existed between SF-8 physical parameters and patient age or surgery delay, whereas a negative correlation existed between patient age and SF-8 mental parameters (P5.012). Retrograde femoral intramedullary nailing is commonly used in elderly patients due to reliable bone healing, minimal soft tissue damage, and immediate full weight bearing. It also offers a valid alternative to antegrade nailing in femoral shaft fractures.

Dr Neubauer and Mr Leitner are from the Department of Trauma Surgery, Landesklinikum Waldviertel Horn, Horn, Drs Krawany and Wagner are from the Department of Trauma Surgery, Wilhelminenspital der Stadt Wien, Vienna, Dr Karlbauer is from Trauma Hospital Salzburg, Salzburg, and Dr Plecko is from Trauma Hospital Graz, Graz, Austria, and the Department of Trauma Surgery, University Hospital Zurich, Zurich, Switzerland. Drs Neubauer, Krawany, Karlbauer, Wagner, and Plecko and Mr Leitner have no relevant financial relationships to disclose. Correspondence should be addressed to: Thomas Neubauer, MD, Department of Trauma Surgery, Landesklinikum Waldviertel Horn, Spitalgasse 10 A-3850 Horn, Switzerland ([email protected]). doi: 10.3928/01477447-20120525-24

JUNE 2012 | Volume 35 • Number 6

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B Figure: Preoperative anteroposterior (left) and lateral (right) radiographs of a 66-year-old man who sustained an AO 33-C2 fracture in a fall from a height (A). Postoperative anteroposterior (left) and lateral (right) radiographs showing that the fracture lines extending into the joint were fixed by a percutaneous lag screw (asterisk) prior to nail insertion (B).

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etrograde intramedullary nailing is a well-accepted stabilization technique for fractures of the diaphyseal and supracondylar areas of the femur.1 As with other minimally invasive techniques, the preservation of local vascularity and soft tissues helps improve results and reduce high complication rates of open reduction and internal fixation (ORIF) procedures in the distal femur.2 Minimal tissue dissection and stable fixation make the implant a popular choice in the elderly because prolonged immobilization is avoided and the risk of fatal complications is reduced. Retrograde intramedullary nailing represents a valuable alternative to antegrade femoral nailing when in situ implants or pathologies of the proximal femur interfere with an antegrade technique.3,4 However, although up to 85% of all supracondylar femoral fractures occur in the elderly,5 reports of outcome and functional results of retrograde intramedullary nailing in this age group are infrequently found in the literature. The purpose of this study was to investigate the functional outcome of femoral retrograde intramedullary nailing in patients older than 60 years with distal femoral and femoral shaft fractures, which to the authors’ knowledge has not yet been published in a clinical investigation.

Materials and Methods Between 2000 and 2008, thirty-five patients (36 femoral fractures) older than 60 years underwent stabilization with retrograde intramedullary nailing. Mean patient age was 81.3 years (range, 60-103 years). Twenty-seven (77.1%) patients were women and 8 (22.9%) were men. All fractures were treated in 2 urban trauma centers and represented acute injuries (n531), nonunited fractures fixed with a prior osteosynthesis method (n53), and pathologic fractures (n52). Fractures were classified according to the AO classification system as 33 (distal metaphyseal area) (n521; 58.3%) and 32 (femoral shaft)

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(n515; 41.7%). The right side was affected in 17 (47.2%) fractures and the left side in 19 (52.8%). Simple fracture types were predominantly found in the supracondylar area (A1, n55; A2, n57; A3, n53; C1, n51; C2, n54; pathologic, n51) and in shaft fractures (A1, n56; A2, n51; A3, n52; B1, n55; pathologic, n51). Depending on the institution, 2 types of retrograde nails were used for fracture stabilization: the Distal Femoral Nail (DFN) (Synthes International, Zuchwil, Switzerland) (n517; 47.2%) and the Supracondylar Unreamed Femoral Nail (sc-UFN) (Synthes Austria, Salzburg, Austria) (n519; 52.8%), a modification of the conventional unreamed femoral nail for use in the distal femoral area (Figure 1).

1 Figure 1: Photograph of the Supracondylar Unreamed Femoral Nail (Synthes Austria, Salzburg, Austria), a modification of the unreamed femur nail for use in the supracondylar area, with additional locking options in the oblique direction.

Surgical Technique Patients were positioned supine with the leg flexed at 40° to 60° with a bolster supporting the distal femur to facilitate reduction. In distal C-type (AO 33) fractures, stabilization of the articular block was accomplished percutaneously prior to nail insertion (Figure 2). A medial paraligamenteous (n530; 83.3%) or transligamenteous (n55; 13.9%) approach was used to open the knee joint; in 1 patient, no patellar ligament was present after knee exarticulation. The entry point of the nail was determined under fluoroscopic control, and arthroscopic assistance was not used. The distal metaphysis was opened using a cannulated reamer over a guide wire, while diaphyseal reaming was performed only in cases of a narrow medullary canal (n52; 5.6%) and in 1 (2.8%) case of revision surgery. Reduction was performed in 28 (77.8%) fractures by ligamentotaxis alone using manual traction (n524) or a transtibial wire (n52), a small reduction table (n51), and an external fixator (n51). In 8 (22.2%) fractures, direct manipulation at the fracture site was performed, and in 2 (5.6%) fractures, an additional cerclage band was applied. All nails were locked on both ends; at the distal end, 27 (75%)

2A

2B Figure 2: Preoperative anteroposterior (left) and lateral (right) radiographs of a 66-year-old man who sustained an AO 33-C2 fracture in a fall from a height (A). Postoperative anteroposterior (left) and lateral (right) radiographs showing that the fracture lines extending into the joint were fixed by a percutaneous lag screw (asterisk) prior to nail insertion (B).

nails were locked by bolts alone and 9 (25%)—more than half of the implanted DFN nails—by an additional spiral blade. Postoperatively, mobilization and physiotherapy started immediately. Weight bearing was allowed depending on fracture type, estimated quality of osteosynthesis by the surgeon, and pattern of concomitant injuries. Patients were followed up until

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Retrograde femoral Nailing in Elderly Patients | Neubauer et al

fracture healing with regular physical and radiographic evaluations. In a retrospective evaluation, patients were assessed using the Lysholm score6 and the SF-8 health questionnaire. If patients were not able to be present for a follow-up examination, local doctors and medical facilities took the information and reported to the authors’ institutions. Statistical analysis was performed with SPSS version 17.0 software (SPSS, Inc, Chicago, Illinois). Categoral variables (eg, sex) were expressed as frequencies and percentages, and continuous variables (eg, age, procedure duration, and scores) were expressed as mean6SD. Differences between study groups were evaluated using the chi-square test for qualitative variables, and unpaired t test was used for quantitative variables. P,.05 was considered statistically significant.

Results The majority of acute fractures represented isolated injuries in low-energy trauma (34/36; 94.4%), whereas 2 (5.6%) occurred in polytraumatized patients. Twenty-nine (80.6%) fractures were caused by simple falls, 3 (8.3%) by a fall from a height, and 2 (5.6%) by traffic accidents; in 2 (5.6%) pathologic fractures, no trauma was detectable. Regarding preinjury mobility status, 3 (8.6%) of 35 patients were bedridden and 2 (5.7%) were able to transfer to a chair or wheelchair with assistance. Three (8.6%) patients were able to walk with assistance, 12 (34.2%) were mobile by themselves using a walking aid, and 15 (42.8%) had unlimited mobility. In 22 (61.1%) of 36 limbs, local anomalies were found that contributed to the choice of retrograde intramedullary nailing for fixation. Most of these specific findings represented implants in situ (15/36; 41.7%) in the proximal (9/22) and distal (6/22) femur, along with contractures of the knee joint in 3 cases, deformities of the proximal femur in 2, and status postknee amputation in 2.


Mean time from injury to surgery was 1.6 days, and mean duration of surgery was 91.5 minutes (Table). Mean start of postoperative mobilization was 1.6 days, and full weight bearing occurred at a mean of 3.3 weeks postoperatively. Fracture healing occurred at a mean of 11.3 weeks postoperatively (Table). No significant differences regarding these parameters were observed between distal femur and femoral shaft fractures. All fractures healed except for 1 (2.8%) septic nonunion in the distal femoral fracture group. A subgroup of periprosthetic fractures (n57) revealed showed no significant differences, except for a significantly later onset of mobilization (P5.03) (Table). Overall complication rate was 30.5% (11/36). Five (13.9%) surgical complications required reintervention. The most common complication was loosening of distal locking bolts (n53), and periprosthetic fracture after 15 months, septic nonunion, subcutaneous bleeding, and recurvation of .5° were observed in 1 patient each. Other complications included cardiac problems (n53) and septic multiple organ failure not related to retrograde intramedullary nailing (n51). Complications occurred more frequently in the distal femoral fracture group (10/21 vs 1/15; P5.009), which included most of the revisions (n54) and pathologic fractures (n55). However, patients with and without complications revealed no significantly differences in duration of surgical procedure (P5.21). Twenty-two (62.8%) of 35 patients had a mean follow-up of 16.5 months (mean of distal femoral fracture group, 13.7 months; mean of femoral shaft fracture group, 20.5 months; P5.27) and were investigated using the Lysholm score and SF-8 questionnaire. In 3 patients who lived in a nursing facility, a short observation period of 3 months was accepted. Radiographic follow-up was limited to 13 (59.1%) patients because 2 lived abroad, 2 refused radiographs, and 5 lived in a nursing facility and were investigated there.

In the examined patient group, mobility status remained at the same level in 18 (81.8%) patients, got worse in 3 (13.6%), and increased in 1 (4.5%). Mean Lysholm score was 76.2 points and revealed no significant differences among different fracture types (78.1 in the femoral shaft fracture group vs 74.9 in the distal femoral fracture group; P5.69). Periprosthetic fractures accomplished relatively good results (mean, 84.8 points), but with no significant difference between other patients (P5.23). Altogether, Lysholm score was good in 8 (36.4%) patients, fair in 11 (50.0%), and poor in 3 (13.6%). The latter were related to multiple joint contractures after poliomyelitis, deficits after septic nonunion, and complications after revision surgery following failed Less Invasive Stabilization System fixation, respectively. In the majority of patients, the encountered functional deficits in Lysholm score were associated with limping or difficulty with weight bearing, stair climbing, or squatting in 13 (59.1%) patients and was related to the general status of their locomotor system (Figure 3). However, most patients (n517; 77.3%) had a stable and painless knee with no swelling, and 3 (13.6%) patients reported some type of locking of the joint. No significant differences among fracture groups were observed in the physical component (38.5 vs 40.5; P5.7) and the mental component (42.3 vs 46.3; P5.49) of the SF-8 score. In the periprosthetic fracture group, a significant increase in SF-8 physical component existed (49.5 vs 39.3; P5.026). No correlation was found between age and time from injury to surgery or age and physical component score. However, a significant negative correlation existed between age and mental component score (Pearson correlation coefficient: 20.523; P5.012), indicating a decrease of mental component score with age.

Discussion In elderly patients, fractures of the femoral supracondylar region are pre-

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Table

Femoral Fractures Treated With 2 Types of Retrograde Nail Mean6SD (Range) Parameter Patient age, y

Total (N536)

Distal Femoral Fracture (n521)

Femoral Shaft Fracture (n515)

P

Periprosthetic Fracture (n57)

P

81.3611.5 (60-103)

80.3611.9 (64–102)

82.9611.4 (60-103)

.516

1.662.3 (0-11)

1.5361.7 (0-11)

1.7163.0 (0 -5)

.832

1.2961.7 (0-5)

.682

91.5631.9 (26-195)

89.1629.4 (26-143)

94.9635.8 (48-195)

.592

85.2616.6 (65-115)

.573

Start of postop mobilization, d

1.661.0 (0 - 5)

1.456.8 (0-2)

1.8761.1 (0-5)

.22

2.060.0 (0-5)

.03

Time to full weight bearing, wk

3.363.2 (0-12)

4.063.8 (0-12)

2.362.0 (0-6)

.09

3.462.0 (1-6)

.88

Time to osseous healing, wk

11.3a61.8 (9-16)

10.761.2a (9-12)

11.762.1 (9-16)

.285

11.861.3 (10-14)

.393

Follow-up, mo

16.5613.9 (3-54)

13.767.9 (3-28)

20.5619.5 (3-54)

.27

15.7621.6 (3-54)

.89

Lysholm score

76.2617.9 (15-93)

74.9621.2 (15-93)

78.1612.8 (50-93)

.692

84.867.4 (75-93)

.232

ROM, deg

102614.7 (80-120)

96.7613.7 (80-120)

110614.1 (90-120)

.174

105 612.9 (90-120)

.629

Physical

39.3611.8 (17.3-57.3)

38.5613.2 (17.3 -57.3)

40.5610.2 (27.9 -54.1)

.7

49.466.3 (39.2-54.1)

.026

Mental

43.9612.9 (24.9-61.7)

42.3613.2 (25.9-61.7)

46.3612.7 (24.9-59.1)

.49

48.3613.3 (26.3-60.4)

.401

Time from injury to surgery, d Duration of surgery, min

81.0611.5 (64 -97)

.924

SF-8

Abbrevations: postop, postoperative; ROM, range of motion. a One case of septic nonunion required final revision surgery.

dominantly caused by low-energy trauma. Although associated injuries and considerable soft tissue damage are infrequently found in this age group, these fractures are difficult to treat due to bad bone stock and impaired vascularity. Conservative treatment is at risk for worse functional outcome (up to 3 times higher than in surgical treatment) and even fatal complications; therefore, it is indicated in undisplaced fractures and patients with high perioperative risks due to comorbidities.7 However, ORIF procedures are associated with high rates of severe complications, especially infections and nonunions,8,9 and, subsequently, a significant decrease of function in this specific age group.10

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The introduction of minimal invasive and soft tissue–preserving techniques, such as retrograde nailing or minimally invasive plate osteosynthesis techniques, helped reduce the high complication rates of ORIF procedures.2 From a biomechanical point of view, retrograde intramedullary nailing represents a reliable and stable fixation method providing results comparable with plate systems regarding construct stability.11 However, some authors report biomechanical advantages of retrograde intramedullary nailing in certain indications that affect predominantly elderly patients with periprosthetic fractures around knee prostheses.12 Others report the superior axial

stability of intramedullary devices in osteoporotic bone of ambulatory patients.13 Clinical outcome results of retrograde intramedullary nailing in the treatment of distal femoral fractures are similar to minimal invasive plating,14 but in minimally invasive plate osteosynthesis, special complications may arise due to the atrophic tissues of a senior population, such as implant loosening in osteoporotic bone and alteration of metaphyseal soft tissues by the bulky implant.15 Frankhauser et al16 reported irritation of the iliotibial tract by the implant as the main cause for implant removal in 23.4% of cases. Therefore, the current authors consider the described intramedullary stability of retrograde intra-



3A

3C

3D

3E

3G

3B

3F

3H

Figure 4: Graphs showing Lysholm scores for limp (A), support (B), stair climbing (C), squatting (D), pain (E), instability (F), swelling (G), and locking (H) in 22 patients at a mean 16.5-month follow-up. Abbreviation: pts, points.


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4A

4B Figure 4: Preoperative anteroposterior (left) and lateral (right) radiographs of a 79-year-old woman who sustained a comminuted fracture AO 33-A3) above a total knee arthroplasty in a simple fall. The implant design was adequate, and no signs of implant loosening were observed (A). Postoperative anteroposterior (left) and lateral (right) radiographs after fracture stabilization with retrograde intramedullary nailing (B).

medullary nailing in combination with less soft tissue irridation preferable in A-type fractures and simple C-type fractures of the supracondylar area of elderly patients. The superior axial stability of intramedullary devices11,13 is especially valuable in situations in which patients cannot follow a staged mobilization protocol postoperatively. Thus, failed postoperative partial weight bearing is frequently reported in up to 50% of cases in the literature.5,17,18 The combination of stable fixation and minimal damage of soft tissues generally yields good results in retrograde intramedullary nailing. Thus, even in preexisting problematic local conditions, such as

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nonmbulatory patients,19 pathologic fractures,20 or elderly patients,14,18,21-23 early mobilization and reliable osseous healing can be achieved. In the current series, only 1 case of nonunion following a septic complication was observed in a patient with extreme obesity, diabetes mellitus, and marked artherosclerosis. Janzing et al23 reported predominantly good and excellent results (88.9%) and no failures in Neer score in patients older than 65 years treated with a Green Seligson Henry nail. Christodoulou et al14 used the Schatzker and Lambert criteria and reported 51% excellent, 9% fair, and 9% poor results after 28 months. The high percentage of fair results in the current series seems to be influenced by the high average age of the patients and the use of a strict knee rating system in which less favorable results are inherent in advanced age. Thus, the majority of the current patients’ score deficits were related to limitations of motion associated with limping, squatting, or stair climbing. However, a minority of patients reported pain, instability of the knee joint, and swelling. Only 3 follow-up patients reported decreased mobility compared with preoperative status. Similarily, El-Kawy et al22 reported no alteration of patient satisfaction even in the presence of problems, citing a 39.2% rate of malalignment but only 10% of patients reporting complaints. Although Gynning and Hansen17 reported backing out of the distal locking bolts as the most common complication (5/30) and reported 2 broken nails, all patients regained flexion of at least 90° with no tenderness at the fracture site. Loosening of distal bolts in osteoporotic bone is a major problem and is subject to various attempts to improve the purchase of the distal screws/bolts.24 In the current series, this was the most frequent single surgical complication. Singh et al25 reported that 10 of 26 distal screws of T2 nails disengaged within 10 weeks in patients aged an average of 78 years. Janzig et al23 reported loosening in 5 of 26 patients and

fixed the distal screws with bone cement due to poor intraoperative purchase in 4 patients. In femoral shaft fractures (AO 33), retrograde intramedullary nailing is an established alternative to antegrade nailing with comparable results.1,4 Almost half of the current patients presented with shaft fractures and revealed local conditions that prevented antegrade nailing in the majority of cases. Over the past few decades, implants in situ became a major issue in preoperative planning due to longer patient lifetimes, an increasing number of joint replacements due to severe osteoarthritis, and an increasing number of femoral fracture stabilization. Thus, Kumar et al18 reported local implants in the femur in 25% of cases, Gynning and Hansen17 in 33% of cases, and the current study in 41.7% of cases. However, the presence of knee arthroplasty does not absolutely prevent retrograde intramedullary nailing because, depending on the design of the prosthesis, the nail can pass the femoral component (Figure 4). The current study had some limitations due to its retrograde design, the relatively small number of patients with follow-up, and the limited observation period of this age group. However, without influence due to fracture type and type of nail used, early mobilization, reliable fracture healing, and a tolerable complication rate were observed with retrograde nailing. Functional deficits were mainly based on mobility limitations related to age. Thus, further studies are required to evaluate the long-term functional outcomes of retrograde intramedullary nailing compared with minimal invasive plating to establish treatment strategies acceptable for use in an impaired locomotor system.

Conclusion Retrograde intrameduallary nailing is a reliable fixation method for extra-articular and simple intra-articular fractures of the supracondylar area of the femur and is an alternative to antegrade intramedullary nailing in femoral shaft fractures in elder-



ly patients. Stable fracture fixation and minimal soft tissue damage provide early mobilization, which helps to prevent fatal complications and make the implant suitable for this age group. Among distal femur fractures and femoral shaft fractures, no differences in functional outcome were observed, and deficits were predominantly related to the increased age of the patients and concomitant limitations of the locomotor system.

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Operative versus non-operative treatment. J Bone Joint Surg Br. 1996; 78:110-114. 8. Della Torre P, Aglietti P, Altissimi M. Results of rigid fixation in 54 supracondylar fractures of the femur. Arch Orthop Traum Surg. 1980; 97:177.1-183.1. 9. Merchan E, Maestu P, Blanco R. Blade-plating of closed displaced supracondylar fractures of the distal femur with the AO system. J Trauma. 1992; 32:174-178. 10. Karpman RR, Del Mar NB. Supracondylar femoral fractures in the frail elderly, fractures in need of treatment. Clin Orthop Relat Res. 1995; 316:21-24. 11. David SM, Harrow ME, Peindl RD, Frick SL, Kellam JF. Comparative biomechanical analysis of supracondylar femur fracture fixation: locked intramedullary nail versus 95-degree angled plate. J Orthop Trauma. 1997; 11:344-350. 12. Bong MR , Egol KA, Koval KJ, Kummer FJ, Su ET, Iesaka K. Comparison of the LISS and a retrograde-inserted supracondylar intramedullary nail for fixation of a periprosthetic distal femur fracture proximal to a total knee arthroplasty. J Arthroplasty. 2002; 17:876-881. 13. Wähnert D, Hoffmeier K, Fröber R, Hofmann GO, Mückley T. Distal femur fractures of the elderly. Different treatment options in a biomechanical comparison. Injury. 2011; 42:655-659. 14. Christodoulou A, Terzidis I, Ploumis A, Metsovitis S, Koukoulidis A, Toptsis C. Supracondylar femoral fractures in elderly patients treated with the dynamic condylar screw and the retrograde intramedullary nail: a comparative study of the two methods. Arch Orthop Trauma Surg. 2005; 125:73-79.

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15. Smith TO, Hedges C, MacNair R, Schankat K, Wimhurst JA. The clinical and radiological outcomes of the LISS plate for distal femoral fractures: a systematic review. Injury. 2009; 40:1049-1063.

7. Butt MS, Krikler SJ, Ali MS. Displaced fractures of the distal femur in elderly patients.

16. Frankhauser F, Gruber G, Schippinger G, et al. Minimal-invasive treatment of distal fem-


oral fractures with the LISS (Less Invasive Stabilization System). Acta Orthop Scand. 2004; 75:56-60. 17. Gynning JB, Hansen D. Treatment of distal femoral fractures with intramedullary supracondylar nails in elderly patients. Injury. 1999; 30:43-46. 18. Kumar A, Jasani V, Butt MS. Management of distal femoral fractures in elderly patients using retrograde titanium supracondylar nails. Injury. 2000; 31:169-173. 19. Cass J, Sems A. Operative versus nonoperative management of distal femur fracture in myelopathic, nonambulatory patients. Orthopedics. 2008; 31:1091-1095. 20. Scholl BM, Jaffe KA. Oncologic uses of the retrograde femoral nail. Clin Orthop Relat Res. 2002; 394:219-226. 21. Armstrong R, Milliren A, Schrantz W, Zeliger K. Retrograde interlocked intramedullary nailing of supracondylar distal femur fractures in an average 76-year-old patient population. Orthopedics. 2003; 26:627-629. 22. El-Kawy S, Ansara S, Moftah A, Shalaby H, Varughese V. Retrograde femoral nailing in elderly patients with supracondylar fracture of the femur; is it the answer for a clinical problem? Int Orthop. 2007; 31:83-86. 23. Janzig HMJ, Stockman B, Van Damme G, Rommens P, Broos PLO. The retrograde intramedullary supracondylar nail: an alternative in the treatment of distal femoral fractures in the elderly? Arch Orthop Trauma Surg. 1998; 118:92-95. 24. Tejwani NC, Park S, Iesaka K, Kummer F. The effect of locked distal screws in retrograde nailing of osteoporotic distal femur fractures: a laboratory study using cadaver femurs. J Orthop Trauma. 2005; 19:380383. 25. Singh SK, El-Gendy KA, Chikkamuniyappa C, Houshian S. The retrograde nail for distal femoral fractures in the elderly: high failure rate of the condyle screw and nut. Injury. 2006; 37:1004-1010.

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