The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 85, Issue 6

Scientific Article | June 01, 2003

Rotational Stability of a Modified Step-Cut for Use in Intercalary Allografts

Drew H. Van Boerum, MD; R. Lor Randall, MD; R. Alexander Mohr, MD; Ernest U. Conrad, MD; Kent N. Bachus, PhD

Investigation performed at the Department of Orthopedics, University of Utah School of Medicine, Salt Lake City, Utah

Drew H. Van Boerum, MD
R. Alexander Mohr, MD
Department of Orthopedics, University of Utah School of Medicine, Salt Lake City, 50 North Medical Drive, UT 84132

R. Lor Randall, MD
Sarcoma Service, Huntsman Cancer Institute, 2000 Circle of Hope, Suite 2100, Salt Lake City, UT 84112. E-mail address: r.lor.randall@hsc.utah.edu

Ernest U. Conrad, MD
Department of Orthopaedics, University of Washington Medical Center, Box 356500, 1959 Pacific Street N.E., Seattle, WA 98195

Kent N. Bachus, PhD
Orthopaedic Bioengineering Research Laboratory, Department of Orthopaedics, 20 South 2030 East, Room 190BPR, University of Utah, Salt Lake City, UT 84112

None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated. Biomet, Warsaw, Indiana, and Brett D. Parkin, Med Tech Medical, Salt Lake City, Utah, donated the femoral nails and loaned the insertion instrumentation. In addition, Musculoskeletal Transplant Foundation, Little Silver, New Jersey, donated the cadaver test specimens.

J Bone Joint Surg Am, 2003 Jun 01;85(6):1073-1078

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Abstract

Background: Intercalary allografts are used for the reconstruction of major skeletal defects. Step-cuts help to provide rotational stability when intramedullary fixation is used. A modified step-cut is proposed to reduce rotation at the interface. This study compares the rotational stability of conventional and modified step-cuts.

Methods: In Phase I, seven pairs of human cadaveric femora were divided into a conventional step-cut group (left femora) and a modified step-cut group (right femora). All femora were cut transversely at the mid-diaphysis. In the conventional group, a 1-cm step-cut was created in the exact midsagittal plane in both the proximal and distal segments. In the modified group, a 1-cm step-cut was created in the parasagittal plane, leaving 2 mm of additional bone on both the proximal and the distal fragment. Phase II was identical except that in the modified step-cut group only 1 mm of additional bone was left. Smooth femoral nails were then placed after standard reaming. Specimens were tested by fixing the proximal segment and applying ±2 N-m (17.7 in-lb) of torque to the distal segments with ten oscillation cycles. Maximum rotation was measured. The data were analyzed with the paired Student t test.

Results: The average rotation in Phase I was 23.3° for the conventional step-cut group and 3.0° for the 2-mm modified step-cut group; the difference was significant (p < 0.001). Four femora sustained an incomplete fracture during nail insertion. The average rotation in Phase II was 20.6° for the conventional step-cut group and 0.5° for the 1-mm modified step-cut group without any fractures; the difference was significant (p < 0.001).

Conclusions: Step-cut modification that leaves more bone in the sagittal plane provides rigid fixation and significantly more stability than the conventional step-cut technique.

Clinical Relevance: Such a modification should facilitate osseous incorporation of an intercalary allograft while providing the benefits of intramedullary fixation. Excessive modification (>1 mm) can result in a fracture of the graft.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Intercalary allografts are used for the reconstruction of major skeletal defects resulting from tumors or trauma. While structural allografts offer the potential advantage of bioincorporation, allograft fracture and interface nonunion are potential disadvantages. The rate of allograft fracture has been reported to range from 8% to 20%, and the rate of nonunion at the allograft-host interface has been reported to range from 11% to 30% ^1-4 . Accordingly, to optimize the use of allografts, fixation should be rigid to reduce the risk of nonunion, and stress-risers should be avoided within the allograft to minimize the risk of fracture. Osteosynthesis with plates and screws provides optimal interface rigidity but demonstrates an increased risk of fracture ^1,2,5,6 because the screw-holes in an allograft serve as stress-risers. Static intramedullary nails that do not violate the allograft provide optimal prophylaxis against fracture ^1,2,6 . However, rotation at the interface can lead to nonunion ⁵ .

Intramedullary fixation of intercalary allografts is achieved by means of endosteal purchase of cortical bone, which provides little if any rotational control. Step-cuts at the host-allograft junction have been advocated to confer rotational stability ^7-10 . However, considerable rotation can remain at the interface when step-cuts are made with use of conventional techniques ⁵ . We propose the use of a modified step-cut to eliminate the rotation at the interface. The purpose of this in vitro study was to compare the rotational stability of a conventional step-cut with that of a modified step-cut with use of a human cadaveric femur model.

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Fig. 1:: Photograph of the standard step-cut used in both Phases I and II.

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Fig. 2:: Close-up photograph of the modified step-cut used in Phase II. The diameter (in millimeters), represented by d, was divided by two and then 1 mm was added to this value. The 1-cm cut was made accordingly. When creating the step-cuts, bone was removed from the lateral portion of the proximal fragment and from the medial portion of the distal fragment.

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Fig. 3:: Photograph showing the final construct just prior to testing. The step-cuts have been created, the proximal and distal ends have been potted, and the intramedullary rod and infrared light-emitting diodes have been attached. The construct was placed in our mechanical testing machine.

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Fig. 4:: Graph representing the amount of motion measured in degrees (x axis) generated during ten oscillations at the given torque (y axis) of the conventional step-cut for one specimen ("d") during testing in Phase I.

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Fig. 5:: Graph representing the amount of motion measured in degrees (x axis) generated during ten oscillations at the given torque (y axis) of the conventional step-cut for one specimen ("m") during testing in Phase II. Note the change in the scale of the x axis compared with that in Figure 4.

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Fig. 6:: Graph demonstrating the average rotation at the step-cut interface in Phases I and II. In Phase I, the average rotation (and standard deviation) was 23.3° ± 8.7° for the conventional step-cut group and 3.0° ± 4.6° for the 2-mm modified step-cut group. In Phase II, the average rotation was 20.6° ± 3.5° for the conventional step-cut group and 0.5° ± 0.2° for the 1-mm modified step-cut group.

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Fig. 7:: Schematic demonstrating the mechanism of a conventional step-cut compared with that of a modified step-cut. A: With the conventional step-cut, as the intramedullary rod is passed down the canal, no perpendicular compressive force is generated at the interface because of the combined effect of the concentric medullary canals (a similar center of rotation) and the bone loss during the creation of the step-cut due to the width of the saw blade. B: The modified step-cut creates a perpendicular compressive force at the interface because of the limited offset of the medullary canals. An excessive modification can overcompensate and lead to fracture.

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TABLE I: Data on Angular Displacement at Each Step-Cut

Phase I (2 mm)			Phase II (1 mm)
Specimen	Step-Cut (deg)		Specimen	Step-Cut (deg)
Conventional	Modified	Conventional	Modified
a	10.3	0.1	h	22.2	0.2
b	13.1	0.2	i	21.3	0.5
c	25.0	0.6	j	24.6	0.5
d	31.9	11.7	k	23.5	0.7
e	22.2	0.5	l	19.6	0.8
f	29.1	7.5	m	18.4	0.3
g	31.8	0.7	n	14.3	0.4
Average	23.3	3.0	Average	20.6	0.5
Standard deviation	8.7	4.6	Standard deviation	3.5	0.2

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TABLE I: Data on Angular Displacement at Each Step-Cut

Phase I (2 mm)			Phase II (1 mm)
Specimen	Step-Cut (deg)		Specimen	Step-Cut (deg)
Conventional	Modified	Conventional	Modified
a	10.3	0.1	h	22.2	0.2
b	13.1	0.2	i	21.3	0.5
c	25.0	0.6	j	24.6	0.5
d	31.9	11.7	k	23.5	0.7
e	22.2	0.5	l	19.6	0.8
f	29.1	7.5	m	18.4	0.3
g	31.8	0.7	n	14.3	0.4
Average	23.3	3.0	Average	20.6	0.5
Standard deviation	8.7	4.6	Standard deviation	3.5	0.2

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Fresh-frozen human cadaveric femora were used in each of two phases of this study. In each phase, seven pairs of femora were divided into two step-cut groups: a conventional step-cut group (left femora) and a modified step-cut group (right femora). All femora were osteotomized transversely at the mid-diaphysis, which was determined as the midpoint between the most inferior aspect of the lateral condyle and the tip of the greater trochanter.

Specimen Preparation

Phase I

The medial-to-lateral diameter of each mid-diaphyseal segment was measured with use of calipers, and the value was rounded to the nearest 0.5 mm. This value was then divided by two to determine the exact midsagittal plane of the femoral segment. In the conventional step-cut group, the 0.052-in (0.13-cm) saw blade was centered in the midsagittal plane. A 1-cm step-cut was created in this plane with use of an oscillating saw in both the proximal and distal segments ( Fig. 1 ). The lateral portion of the proximal fragment and the medial portion of the distal fragment were removed.

In the modified step-cut group, a sagittal line was drawn 2 mm lateral to the midsagittal plane on the proximal fragment and 2 mm medial to the midsagittal plane on the distal fragment. The saw blade was again centered in the respective parasagittal plane. A 1-cm step-cut was then created accordingly.

After the proximal and distal segments of each femur were reduced, the construct was placed in a vise. A guide-pin was drilled through the piriformis fossa and into the medullary canal. An end-cutting reamer was used over the guide-pin to open the piriformis fossa. A guidewire was then passed antegrade down the medullary canal. The specimens were reamed in standard fashion, increasing in 1-mm increments until substantial endosteal interface vibrations were encountered at the osteotomy site. The canal was then reamed an additional 1 mm in two 0.5-mm increments.

The articular ends of each piece were potted in polyvinylchloride pipe that was 3 in (7.6 cm) long and 4 in (10.2 cm) in diameter. Smooth, slotted femoral nails (Uniflex; Biomet, Warsaw, Indiana) were placed in an antegrade fashion down the canal while the femora were held in a reduced position. The diameter of the nail selected was 1 mm less than the maximum diameter reamed.

Phase II

The conventional step-cut specimens were prepared in exactly the same manner as in Phase I. In the modified step-cut group, a sagittal line was drawn only 1 mm lateral to the midsagittal plane on the proximal fragment and only 1 mm medial to the midsagittal plane on the distal piece ( Fig. 2 ). Once again, the saw blade was centered on this parasagittal plane. A 1-cm step-cut was then created accordingly.

Reduction of the step-cuts, placement of the intramedullary nail, and fixation for testing were performed in a fashion identical to that in Phase I.

Specimen Testing

Following nail insertion, the constructs were rigidly fixed into the upper and lower gimbals of a custom four-axis mechanical testing machine that maintained a constant axial load while applying cyclical axial torque. Both the proximal and distal femoral segments had a rigid-body position marking flag attached in the sagittal plane ( Fig. 3 ). Each flag contained four infrared light-emitting diodes, which in conjunction with the diodes' positional receiving hardware (OPTOTRAK; Northern Digital, Waterloo, Ontario, Canada), provided positional information in the x, y, and z axes. Angular displacement of each construct was calculated from the x, y, and z positional data of the infrared light-emitting diodes that were grouped into two rigid-body marking flags—one superior and one inferior to the cut. Inverse kinematics allowed the creation of a 4 × 4 transform matrix that provided the three-dimensional translation and rotation of each rigid body, thereby enabling calculation of angular displacement. For specimen testing, a constant 50-N axial load was applied. Then, with the proximal segment held stationary, the distal fragment was cycled ten times with use of ±2 N-m (17.7 in-lb) of torque around the intramedullary nail.

Data Analysis

Motion was calculated with use of the data collected from the infrared light-emitting diodes. These data were then plotted in torque-rotation graphs ( Figs. 4 and 5 ). From these graphs, an angular displacement value was calculated for each specimen.

Statistical Methods

The rotational displacement measured from the conventional and modified step-cut specimens in Phases I and II ( Table I ) were compared with use of the paired Student t test. The conventional step-cut data from Phases I and II as well as the data on the two modified step-cut groups from each phase were also compared with each other.

Results

Introduction | Materials and Methods | Results | Discussion | References

Phase I

The average rotation (and standard deviation) was 23.3° ± 8.7° for the seven specimens that had a conventional step-cut and 3.0° ± 4.6° for the seven specimens that had the 2-mm modified step-cut; the difference was significant (p < 0.001) ( Fig. 6 ). Nondisplaced incomplete fractures were noted during nail insertion in one femur in the conventional group and in three femora in the 2-mm modified step-cut group. No femur fractured or demonstrated fracture propagation during mechanical testing. Fracture of the femur during nail insertion was not associated with any difference in the calculated rotation relative to the femora without fracture.

Phase II

The average rotation (and standard deviation) was 20.6° ± 3.5° in the conventional step-cut group and 0.5° ± 0.2° in the 1-mm modified step-cut group; the difference was significant (p < 0.001) ( Fig. 6 ). No fracture was noted in either the conventional or the 1-mm modified group.

No significant difference was demonstrated between the Phase I and II conventional step-cut groups (the difference was 3.2°; p = 0.507) or between the 1-mm and 2-mm modified groups (p = 0.193). However, an important, clinically relevant observation occurred with respect to the number of fractures. Fractures occurred in three of the seven femora in the 2-mm modified step-cut group in Phase I and in none of the femora in the 1-mm modified group in Phase II.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

In 1908, Lexer reported the first large series of allograft bone transplantations ¹¹ . Twenty-three whole joint and eleven hemijoint transplants about the knee were performed. A series of case reports followed, but it was not until the 1960s that multiple reports of large series began to appear. This resurgence of the technique was related to the discovery that the immunogenicity of the graft could be reduced by freezing ^11-14 . In a long-term follow-up study reported in 1973, Parrish demonstrated that the allografts used in twenty-one patients had been partially replaced and incorporated by the host ¹⁵ .

Since these early reports, many studies have demonstrated the success of allograft replacement for the treatment of osseous defects resulting from tumors ^1-4,11,15-17 . There are also numerous reports on the use of allografts in conjunction with endoprosthetic implants for the reconstruction of juxta-articular osseous defects ^7-10 . In 1991, Jaffe et al. ¹⁸ described the use of allografts for the treatment of traumatic skeletal defects, and, in 1994, Vander Griend ¹ reported the results after such treatment in four patients with traumatic skeletal defects. Infection, fracture, nonunion, and immunologic rejection of allografts can all lead to clinical failure.

Fractures

Early or late fractures of the allograft can complicate the postoperative course. The rate of allograft fracture has been reported to range between 8% and 20% ^1-4 . Screw-holes in allografts serve as stress-risers ^1-4 . Most of these fractures occur before the third year ^4,11 . Intramedullary fixation has been shown to substantially reduce the rate of fracture ¹ . Vander Griend ¹ demonstrated that only two (3.5%) of fifty-seven allografts fixed with an intramedullary nail had a fracture, and both fractures occurred through screw-holes in osteoarticular allografts.

Muir and Johnson, in 1995, reported the results of a study in which an intramedullary nail was compared with a dynamic compression plate with respect to fixation of intercalary allografts in a sheep model ⁵ . They identified histologic evidence of incomplete fissure fractures in three of five allografts that were fixed with a dynamic compression plate.

Nonunion

The clinical success of structural allografts depends on the achievement of osseous union at the host-allograft interface. Delayed unions or nonunions are an important source of patient morbidity. The rate of nonunion at the host-allograft interface has been reported to range from 11% to 30% ^1-4 .

Nonunion occurs in up to 26% of diaphyseal interfaces ⁴ . Diaphyseal nonunions are much more common than metadiaphyseal or metaphyseal nonunions ^1,2,4 . Nonunion is more frequently associated with intramedullary fixation than with plate-and-screw fixation ¹ . This finding is thought to be related to the rigidity of the fixation. Motion at the host-allograft interface can lead to nonunion ⁵ . The healing of the host-allograft junction and the integrity of the allograft itself depend on the mode of fixation, which must afford the stability required for union, endure complex loads long enough for healing to occur, and allow functional rehabilitation of the extremity ¹ .

Step-Cuts

A transverse osteotomy can lead to poor rotational stability at the host-allograft junction ⁵ . Step-cuts have been used at host-allograft junctions to provide rotational stability. When intercalary allografts are used in conjunction with intramedullary fixation, interlocking screws are placed only through the host bone, proximal and distal to the allograft. Therefore, the intercalary segment of allograft is free to rotate about the nail. A step-cut provides intrinsic stability of the construct and prevents free rotation of the allograft about the nail. Many studies have suggested that step-cuts can provide rotational stability in composite (allograft and endoprosthetic) reconstructions of the proximal or distal part of the femur ^7-10 .

The biomechanics of rotational stability when the step-cut is used in conjunction with intramedullary fixation have not been studied. To the best of our knowledge, the current study is the first to compare the rotational stability of a conventional step-cut with any proposed modification to improve rotational stability.

This proposed modification of the conventional step-cut provided markedly improved fixation with minimal detectable rotation in the range of axial torque tested. Rotational stability was significantly greater with the modified step-cut than with the conventional step-cut. The conventional step-cut allowed the clearance between the medullary canal and the intramedullary rod to be transformed into relative motion at the step-cut ( Fig. 7 , A ). The mechanism that reduced the relative motion was the creation of a 1 or 2-mm offset (Phase II and I, respectively) at the junction site of the medullary canal in the transverse plane. The intramedullary rod then created a compressive force at the step-cut perpendicular to the long axis as it was forced to contact contralateral sides of the medullary canal of the proximal and distal bone-segments ( Fig. 7 , B ). Overreaming of the canal by 1 mm more than the rod diameter did not introduce enough play to compensate for this interference fit, and it prevented relative motion at the step-cut when transmitting torque. Accordingly, such a modification should facilitate the osseous incorporation of an intercalary allograft while providing the benefits of intramedullary fixation. In Phase I (the 2-mm modified step-cut group), we demonstrated that stability was significantly improved, but three of the seven femora sustained an incomplete fracture during nail insertion. Therefore, the surgeon must be aware of the risk that an excessive modification of =2 mm can result in fracture of the graft. Although such a modification provides rigid fixation, it does not ensure incorporation of the allograft, as biologic and immunologic variables may still preclude osseous integration.

References

Introduction | Materials and Methods | Results | Discussion | References

Vander GriendRA. The effect of internal fixation on the healing of large allografts. J Bone Joint Surg Am,1994;76: 657-63. 76657 1994 [PubMed]

Ortiz-Cruz E, Gebhardt MC, Jennings LC, Springfield DS,Mankin HJ. The results of transplantation of intercalary allografts after resection of tumors. A long-term follow-up study. J Bone Joint Surg Am,1997;79: 97-106. 7997 1997 [PubMed]

MankinHJ, Gebhardt MC, Jennings LC, Springfield DS,Tomford WW. Long-term results of allograft replacement in the management of bone tumors. Clin Orthop,1996;324: 86-97. 32486 1996 [PubMed][CrossRef]

CaraJA, Laclériga A,Cañadell J. Intercalary bone allografts. 23 tumor cases followed for 3 years. Acta Orthop Scand,1994;65: 42-6. 6542 1994 [PubMed][CrossRef]

MuirP,Johnson KA. Tibial intercalary allograft incorporation: comparison of fixation with locked intramedullary nail and dynamic compression plate. J Orthop Res,1995;13: 132-7. 13132 1995 [PubMed][CrossRef]

BeneveniaJ, Zimmerman M, Keating J, Cyran F, Blacksin M,Parsons JR. Mechanical environment affects allograft incorporation. J Biomed Mater Res,2000;53: 67-72. 5367 2000 [PubMed][CrossRef]

MarkelMD, Wood SA, Bogdanske JJ, Rapoff AJ, Kalscheur VL, Bouvy BM, Rock MG, Chao EY,Vanderby R Jr. Comparison of allograft/endoprosthetic composites with a step-cut or transverse osteotomy configuration. J Orthop Res,1995;13: 639-41. 13639 1995 [PubMed][CrossRef]

MoranMC. Postoperative fracture of the host femur in allograft-prosthetic reconstruction of the hip. J Arthroplasty,1995;10: 550-3. 10550 1995 [PubMed][CrossRef]

WongP,Gross AE. The use of structural allografts for treating periprosthetic fractures about the hip and knee. Orthop Clin North Am,1999;30: 259-64. 30259 1999 [PubMed][CrossRef]

NambaRS, Brick GW,Murray WR. Revision total hip arthroplasty with correctional femoral osteotomy in Paget's disease. J Arthroplasty,1997;12: 591-5. 12591 1997 [PubMed][CrossRef]

MankinHJ, Doppelt S,Tomford W. Clinical experience with allograft implantation. The first ten years. Clin Orthop,1983;174: 69-86. 17469 1983 [PubMed]

HerndonCH,Chase SW. The fate of massive autogenous and homogenous bone grafts including articular surfaces. Surg Gynecol Obstet,1954;98: 273-90. 98273 1954 [PubMed]

BonfiglioM, Jeter WS,Smith CL. The immune concept: its relation to bone transplantation. Ann NY Acad Sci,1955;59: 417-32. 59417 1955 [PubMed][CrossRef]

Curtiss PHJr, Powell AE,Herndon CH. Immunological factors in homogenous-bone transplantation. III. The inability of homogenous rabbit bone to induce circulating antibodies in rabbits. J Bone Joint Surg Am,1959;41: 1482-8. 411482 1959 [PubMed]

ParrishFF. Allograft replacement of all or part of the end of a long bone following excision of a tumor. J Bone Joint Surg Am,1973;55: 1-22. 551 1973 [PubMed]

MakleyJT. The use of allografts to reconstruct intercalary defects of long bones. Clin Orthop,1985;197: 58-75. 19758 1985 [PubMed]

SalaiM, Rahamimov N, Pritch M, Rotstein Z,Horoszowski H. Massive bone allografts in the treatment of pathologic fractures due to bone metastases. J Surg Oncol,1997;66: 93-6. 6693 1997 [PubMed][CrossRef]

JaffeKA, Morris SG, Sorrell RG, Gebhardt MC,Mankin HJ. Massive bone allografts for traumatic skeletal defects. South Med J,1991;84: 975-82. 84975 1991 [PubMed][CrossRef]

Topics

fracture ; femur ; allografting ; intramedullary rods

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Drew H. Van Boerum, R. Lor Randall, R. Alexander Mohr, Ernest U. Conrad, Kent N. Bachus; Rotational Stability of a Modified Step-Cut for Use in Intercalary Allografts. The Journal of Bone & Joint Surgery. 2003 Jun;85(6):1073-1078.

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