Design and application of combined hip-knee intramedullary joint replacements

The Journal of Arthroplasty Vol. 14 No. 8 1999

Design and Application of Combined Hip-Knee Intramedullary Joint Replacements P e t e r S. W a l k e r ,

PhD,* Wai Weng

George

FRCS,{

Bentley,

and

Yoon, BSc,* Stephen Sarah

R. C a n n o n ,

K. M u i r h e a d - A l l w o o d ,

FRCS,t

FRCS§

Abstract" Cases in which there is a total hip arthroplasty and a stemmed total knee arthroplasty in the same femur, with loosening of 1 or both components, with serious endosteal bone loss or even a fracture between the stems present a difficult reconstruction problem. We describe a reconstruction using a combined hip and stemmed knee, designed so that they could be rigidly connected during the surgical procedure. The advantages of this implant design are that the entire femur with its muscle attachments is preserved, and the inherent stability allows for immediate weight bearing. To determine the viability of the connection between the hip and the knee, a stress analysis was carried out using finite element analysis. Guidelines were thus provided for the required metal and cement thicknesses. Three case examples are presented with an average-follow-up of 3 years. It was shown that the combined hip-knee implant could provide successful results for these difficult reconstructive problems in appropriately selected cases. Key words: combination hip-knee total joint, hip arthroplasty, knee arthroplasty, massive prostheses, finite element analysis.

has b e e n well recognized that in hinged knee replacements in which the stem tip is initially adjacent to the endosteal surface, loosening and fracture frequently occur [ 1]. It is standard orthopaedic practice to extend fixation of revision steins b e y o n d the area of b o n e destruction and endosteal resorption to obtain sufficient fixation of the revised implant. If b o t h hip and knee replacements are present, this practice can lead to close proximity of the revision stems and increase the chance of a fracture of the bone. W h e n intramedullary stems are closely applied, a n y revision situation does not allow increased length to be gained. In cases in which fracture has occurred, conservative treatm e n t is usually preferable [2,3], but it is well recognized that in the presence of poor b o n e stock, particularly w h e n the endosteal surface is covered with cement, fracture healing is unpredictable, and fixation of external plates, either by screws or by circumferential cramping, is fraught with difficulties. The older patient m a y be unable to e n d u r e prolonged periods of n o n - w e i g h t bearing or bed rest that m a y be required to obtain healing.

Hip and k n e e r e p l a c e m e n t within an ipsilateral f e m u r is an increasingly recognized orthopaedic occurrence. Revision for loosening of either of these c o m p o n e n t s can lead to difficult reconstructive problems, which m a y be associated with loss of b o n e stock and fracture. This reconstruction can be particularly difficult if a k n e e r e p l a c e m e n t with an intramedullary s t e m has b e e n previously used. The loosening process usually results in loss of endosteal bone, which m a y be, on occasion, severe, and the s t e m of either hip or k n e e can p e n e t r a t e the cortical area of b o n e leading to a stress-riser and fracture. It From the *Centre for Biomedical Engineering, University College London, Royal National Orthopaedic Hospital Trust, Stanmore; ~Royal National Orthopaedic Hospital Trust, Stanmore; ~Institute of Orthopaedics, Royal National Orthopaedic Hospital Trust Stanmore; and §The Whittington Hospital, London, and Royal National Orthopaedic Hospital Trust, Stanmore, United Kingdom. S u b m i n e d September 9, 1998; accepted April 26, 1999. No benefits or f u n d s were received in supporl of the study. Reprint requests: Peter S. Walker, PhD, Centre for Biomedical Engineering, University College London, RNOHT Stanmore, Middlesex HA7 4LP, UK. Copyright © 1999 by Churchill Livingstone ® 0883- 5 4 0 3 / 9 9 / 1 4 0 8 - 0 0 0 9 5 1 0 . 0 0 / 0

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The c o m b i n e d h i p - k n e e intramedullary joint rep l a c e m e n t was designed to address the a f o r e m e n tioned situations by providing i m m e d i a t e stability, allowing early mobilization. The implant consists of hip and knee stems that are rigidly connected at surgery. The aims of the present study w e r e to d e t e r m i n e the viability of the implant regarding the strength of the connection b e t w e e n the stems, the preservation of the bone, and the functional possibilities. A stress analysis was carried out to determ i n e guidelines for the dimensions of the stems and c e m e n t mantle, and the clinical sequelae are illustrated from follow-up of 3 patients. This article presents a s h o r t - t e r m analysis that concludes that the m e c h a n i s m is sound and will r e m a i n intact in the short-term. To date, we have not recognized a n y m a j o r p r o b l e m with b o n e resorption or loosening, but a longer follow-up is required to address these problems. This article therefore represents an early report outlining the feasibility of the m e t h o d .

Materials and M e t h o d s

Implant Design and Stress Analysis The femoral c o m p o n e n t was c u s t o m - m a d e to be a close fit to the canal in the proximal femur. This region was grooved, grit blasted, and h y d r o x y a p a tite coated. The distal part of the stem was smooth. The intramedullary stems of the knee hinge were also titanium alloy, again grit blasted and h y d r o x y apatite coated for u n c e m e n t e d applications. The k n e e hinge-bearing c o m p o n e n t s w e r e m a d e from c o b a l t - c h r o m e alloy a n d u l t r a - h i g h - m o l e c u l a r weight polyethylene. A t h r e e - d i m e n s i o n a l finite e l e m e n t model was formulated based on a typical design from the clinical cases (Fig. 1). The dimensional p a r a m e t e r s w e r e the stem diameter (10, 12, and 14 m m ) , the c e m e n t mantle thickness (2, and 4 ram), and the wall thickness of the outer tube (1, 2, and 3 ram). Analyses were carried out for all combinations of these parameters, which w e r e chosen to be within a feasible range to fit typical femora. One further m o d e l was used to investigate the effect of debonding on the c e m e n t - m e t a l interfaces. The finite e l e m e n t package used was COSMOS, which was capable of linear and nonlinear solutions, the latter being used to investigate the effect of debonding of interfaces. The properties of the materials w e r e titanium alloy, m o d u l u s of elasticity E = i06,000 MPa, Poisson's ratio v = 0.33; acrylic bone cement, m o d u l u s of elasticity E = 2,700 MPa, Poisson's ratio v = 0.33. The materials w e r e ass u m e d to be h o m o g e n e o u s , isotropic, and linearly elastic.

A

AXIAL FORCE FRONTAL stem 0,ameter

cement thickness wall --~ thickness acrylic ~ cement

~

.~

tubular part - " " of knee femoral component

~ "1

)

" acrylic

spacer (optional)

> .........

M base fixed

Fig. 1. (A) A schematic of the combined hip-knee intramedullary joint replacement. The model of the hip-knee connection is shown for the finite element analysis. (B) The finite element model showing the quadrilateral elements. The white elements represent the cement.

Combined Hip-Knee Replacements Regarding the forces and m o m e n t s that occur in the diaphysis of the femur, data have b e c o m e available from a telemetrized distal femoral replacem e n t [5,6]. For level walking, the axial force was 1,500 N, whereas the bending m o m e n t s in the frontal and sagittal planes peaked simultaneously at 50 and 30 N . m. These values were measured at 230 m m from the distal end of the femur. The level of interest for our h i p - k n e e implant is at the connection b e t w e e n the stem and the tube, 120 m m being representative. Assuming the m o m e n t s reduce linearly to 0 at the knee [4], the resultant m o m e n t at the 1 2 0 - m m level was calculated to be 30 N . m . Hence, a 1,500-N axial force and a 30-N - in m o m e n t were applied to the model. Eight-noded quadrilateral elements were used. To obtain a satisfactory mesh, analyses were carried out, then the mesh density was progressively increased in the regions of maximal stress gradient, maintaining a satisfactory aspect ratio of the elements, until convergence of the results. The final models consisted of 11,000 elements.

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c o m p o n e n t was designed for proximal cementation, c e m e n t was placed in the proximal region using finger packing after the stem had been inserted partway. Cement was placed inside the tube of the knee c o m p o n e n t as well as a r o u n d the distal end just before final seating. This c o m p o n e n t was then pressed into place. Care was taken at the stage w h e n the tube engaged the tip of the hip c o m p o n e n t to ensure it was central and fully b o t t o m e d out. The tube itself had small acrylic spacers fixed to the inner wall for stem centralization. After the cement had hardened, the hip was reduced. The tibial c o m p o n e n t and knee assembly was then carried out. The assembly was finally checked at both the hip and the knee before closure. Radiographs and clinical examination were carried out postoperatively and at 6 months, 1 year, and yearly after that. General clinical observations were recorded. The radiographs were examined for radiolucent lines a r o u n d the interface and for any signs of change in bone density or m o v e m e n t of the c o m p o n e n t s within the bone.

Surgical Technique and Clinical Analysis The technique varied to some extent for each specific case, but there were certain general features. The exposure was as if performing simultaneous hip and knee replacements. The existing c o m p o n e n t s were r e m o v e d together with all remnants of c e m e n t and fibrous tissue adjacent to the endosteal bone. The n e w hip and knee femoral c o m p o n e n t s were introduced separately to test for access, and further bone was r e m o v e d as required. Attention was given to ensuring that the hip femoral c o m p o n e n t centered in the canal distally. This centering was especially important if the proximal end of the c o m p o n e n t was close fitting for uncem e n t e d use (hydroxyapatite coated). The acetabular c o m p o n e n t was revised, if necessary. The hip c o m p o n e n t was t h e n placed, and trial femoral heads were used to ascertain that the hip was stable. The total knee was assembled and checked for range of motion and patellar tracking. The hip and knee c o m p o n e n t s were then both inserted in that order, and the leg length was measured. In cases in which there had been a midfemoral fracture, restoration of length was necessary. This restoration could be accomplished by placing acrylic spacers inside the tube of the knee c o m p o n e n t . Such spacers were also used w h e n the length was correct, to provide a positive seat w h e n the c o m p o n e n t s were being cemented. The canals were then lavaged before final fixation of the components. The hip c o m p o n e n t was inserted first, again checking for distal centralization. W h e n the

Results Stress Analysis The results were c o m p u t e d as maximal tensile and compressive stresses, so that the values could be compared with k n o w n strength values of the materials [7-10]. The tensile stresses in the stem as a result of bending were at a m a x i m u m w h e r e the stem entered the c e m e n t annulus. The tensile stress for a diameter of 10 m m was found to be less than 60% of the fatigue limit of titanium alloy with the larger-diameter stems showing even lower values. The stresses were minimally affected by increasing the cement or outer tube thickness. The cement stresses are s h o w n in Fig. 2 for a range of variables. As expected, the stem diameter was a major factor, with a 1 0 - m m diameter producing stresses that were excessive in both tension and compression. There was clearly an advantage in maximizing the stem diameter, but only at a 1 4 - m m diameter did the stresses become lower than the fatigue limit. A thicker cement mantle reduced stresses significantly. The compressive strength of cement is likely to be the most important criterion here, as discussed later. The stresses in the outer tube were less than one half those in the stem up to stem diameters of 14 m m and hence w o u l d not be a strength concern. Increasing tube wall thickness > 1 m m reduced tube stresses, but this led to an increase in the cement stresses by increasing the rigidity of the composite.

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The Journal of Arthroplasty Vol. 14 No. 8 December 1999 CEMENT MANTLE THICKNESS ~2mm

~ 4 m m

40-

Z O 30Go z 20 LU

10

TENSILE STRENGTH

-- ~

~

.~

(Burke)

FATIGUE LIMIT (Krause)

MPa 10Z O 20 -

GO

Go 3 0 LU n"

n 40 O 50O

10 mm

pression of 12% and 10%, with similar changes in the stresses of the outer tube. The c e m e n t stresses were changed the most, with a reduction in tensile stress to almost 0, and an increase in compressive stress of 20%.

S "~'~'

ULTIMATE COMPRESSIVE STRENGTH (Saha)

Fig. 2. The maximal tensile and compressive stresses in the cement annulus for stem diameters of 10, 12, and 14 mm. Cement mantle thickness, 2 and 4 ram; outer tube wall thickness, 1 mm.

The increases in c e m e n t stresses w e r e relatively small, however, and did not bring the c e m e n t stresses close to the compressive strength. The analysis of the u n b o n d e d c e m e n t - s t e m interface showed an increase in stem tension and com-

Clinical Cases

Case 1. A 36-year-old w o m a n with r h e u m a t o i d arthritis presented with a fractured f e m u r above the femoral stein after a hinged total k n e e arthroplasty (Fig. 3A). Initial t r e a t m e n t had involved double AO (Synthes, Robert Mathys Co., Bettlach, Switzerland) plating and b o n e grafting, but she w e n t on to h a v e a significant n o n u n i o n with varus deformity of the hip. The hip was severely affected by r h e u m a toid arthritis and was stiff in all modalities of m o v e m e n t . It was decided to treat the patient with a c o m b i n e d h i p - k n e e replacement. A femoral compon e n t of hip r e p l a c e m e n t was designed w h o s e proximal half was coated with h y d r o x y a p a t i t e and designed to press-fit into the femur. The distal part of the stein was grooved to allow c e m e n t fixation within the hollowed c o m p o n e n t of a SMILES rotating hinge knee r e p l a c e m e n t (Stanmore Implants Worldwide, Ltd., UK). The femoral c o m p o n e n t of the hip r e p l a c e m e n t was inserted first, t h e n from the distal f e m u r c e m e n t was inserted a r o u n d the distal portion of the hip stem, and the SMILES rotating hinge femoral c o m p o n e n t was c e m e n t e d onto the stem. The patient had i m m e d i a t e stability

Fig. 3. (A) Preoperative view of a 36-year-old woman shows an unsuccessful attempt to fix fracture (case 1). (B and C) The combined hip-knee implant, using a SMILES rotating hinge (case 1).

Combined Hip-Knee Replacements

in the postoperative period and was able to mobilize in a fully weight-bearing m a n n e r . Radiolucent lines w e r e seen i m m e d i a t e l y postoperatively a r o u n d part of the knee stem because of the condition of the canals after the revision of the k n e e hinge, but these lines were nonprogressive (Fig. 3B and C). The u n c e m e n t e d l o n g - s t e m m e d total hip r e p l a c e m e n t a p p e a r e d stable with no evidence of radiolucent lines or osteolysis proximally, despite the p o o r femoral b o n e stock. The patient had satisfactory function 3 years after insertion. Case 2. A 61-year-old m a n with r h e u m a t o i d arthritis suffered a loosening of a left total hip r e p l a c e m e n t above a loose left S t a n m o r e total k n e e r e p l a c e m e n t (Biomet, UK). As a result of excessive micromotion, a fracture occurred b e t w e e n the 2 c o m p o n e n t s . An initial stabilization was a t t e m p t e d using a M e n n e n plate (C.H. Medical, Exeter, UK) externally, but this failed rapidly (Fig. 4A). A combined revision of the hip and k n e e was decided on. On this occasion, a c e m e n t e d femoral c o m p o n e n t for the hip r e p l a c e m e n t was used. C e m e n t was inserted only after initial stabilization of the stem into the diaphysis of the femur. Once the c e m e n t a r o u n d the hip r e p l a c e m e n t had cured, a further c e m e n t e d technique for the femoral c o m p o n e n t of the k n e e r e p l a c e m e n t was p e r f o r m e d from the distal end of the femur. No radiolucent lines w e r e seen a r o u n d the interface of the hip stem (Fig. 4B). The distal femoral

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c o m p o n e n t displayed a radiolucent line because of the lack of trabecular b o n e on the revised endosteal surface (Fig. 4C), but the line did not progress 15 m o n t h s postoperatively. Overall, there was no sign of progressive loosening at 15 m o n t h s , and the patient was satisfied with the prosthesis f r o m a functional point of view. Case 3. A 64-year-old w o m a n with r h e u m a t o i d arthritis u n d e r w e n t revision of a hip r e p l a c e m e n t with a l o n g - s t e m m e d c e m e n t e d prosthesis. During the initial postoperative period, a fracture of the distal two thirds of the f e m u r occurred. Bone grafting attempts w e r e made, but she was left with a significant n o n u n i o n and p o o r bone stock. The k n e e b e n e a t h the fracture was e x t r e m e l y stiff and showing features of r h e u m a t o i d arthritis (Fig. 5A). On this occasion, the stable c e m e n t e d hip c o m p o n e n t was left in place and a custom-built femoral c o m p o n e n t was designed to allow c e m e n t a t i o n onto the stem in situ. I m m e d i a t e stabilization of the construct was noted, and the patient was able to mobilize in a weight-bearing manner. On this occasion, an adapted K i n e m a x total knee replacement (Stryker-OsteonicsHowmedica, Rutherford, N J) was used (Fig. 5B). Radiographically, there w e r e no signs of loosening or migration of the knee or the hip c o m p o n e n t 4 years after the initial procedure. On e x a m i n a t i o n at 4 years' follow-up, the patient continued to walk well w i t h o u t requiring a n y cane or crutch. The

Fig. 4. (A) A Mennen plate used unsuccessfully to stabilize a fracture between the hip and knee stems (case 2). (B) Proximal cementation of the femoral hip stein (case 2). (C) The distal region using a SMILES rotating hinge (case 2).

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The Journal of Arthroplasty Vol. 14 No. 8 December 1999

Fig. 5. (A) Preoperative anterior view of a distal fracture around a long-stem hip (case 3). (B) Postoperative anterior view showing the same femoral component and a modified Kinemax condylar (case 3).

range of flexion at the knee was 95 ° , and the patient was satisfied with the outcome.

Discussion In general, w h e n designing custom implants, it is not feasible to carry out a complete analysis of each one because of the time and expense involved. Design guidelines can be formulated, however, which generalize the p r o b l e m and which can t h e n be applied to each case. In this case, stresses in the implant components are the most important consideration to avoid a mechanical failure. Calculated stresses can be compared with the mechanical properties of the materials, the fatigue stress being the most relevant. The properties of materials such as acrylic cement and polyethylene vary depending on processing methods [7-9], and bone properties vary over a wide range, w h e r e a s defects in the cortex are c o m m o n in this type of case with u n k n o w n stress-concentrating effects. A n o t h e r p r o b l e m in the analysis is that there are inadequate data of the forces acting on the bones and on the c o m p o n e n t s . Although the forces on the hip itself h a v e b e e n d e t e r m i n e d directly by telemetry, k n e e forces h a v e b e e n d e t e r m i n e d only by indirect calculations. Estimates of the forces on implant c o m p o n e n t s need to account for the level in the femur, on the muscle forces b e t w e e n the hip or k n e e at the required level [4-6], and the mechanical

connection (such as cemented) b e t w e e n the c o m p o n e n t and the bone. In the h i p - k n e e implant, telemetrized data w e r e used of the forces and m o m e n t s acting on the shaft of a distal femoral replacement. In translating these data to the required level, it was assumed that the axial force would be constant in the shaft, which would occur for no s t e m - b o n e bonding. The frontal and sagittal m o m e n t s were calculated to diminish from the hip to the knee, being small at the k n e e itself [4]. W h e n the stresses that were calculated were c o m p a r e d with the material properties, it was clear that the c e m e n t was m o s t at risk. To minimize c e m e n t stresses, the ideal dimensions are 14-ram diameter for the stem, 4 - m m c e m e n t thickness, and 1 - m m tube wall thickness, giving a total of 24 m m outer diameter. The available b o n e canal diameter, however, is usually less t h a n this, requiring c o m p r o mises to be made. A further detail is that the stem centralizers in the metal tube are i m p o r t a n t to avoid a region of small c e m e n t thickness. The indications for combined h i p - k n e e implants require careful consideration, and m o r e conservative approaches m u s t be considered first. Such an implant is appropriate, however, if revision of a loose hip or k n e e would h a v e a p o o r prognosis or if it were not possible to obtain fracture healing b e t w e e n a hip and knee stem. Severe endosteal b o n e loss, possibly after previous revisions or from w e a r particles, is also an i m p o r t a n t consideration pointing toward a h i p - k n e e implant. There are several positive advantages, in particular, the implant allows early weight bearing, and future loosening in the f e m u r is no longer an issue. The surgical procedure m a y only require incisions similar to those for a total hip and total k n e e replacement, but a midthigh incision m a y be needed in cases in which there had b e e n a midshaft fracture. The possibility of r e m o v a l of a h i p - k n e e implant m u s t be considered in case of infection. For this reason, it is an advantage to use c e m e n t or h y d r o x y apatite coating close to the hip and the knee only and not along the entire length of the femur. Stress shielding of the femur is an issue, although if the implant is fixed at the hip and knee regions, both axial and bending forces are transmitted to the femur. In addition, the muscles themselves apply significant forces. Considering the severity of the predisposing condition in which the h i p - k n e e implant is used, the gain in function is likely to be sufficiently i m p o r t a n t to justify the risk of some osteopenia. In a n y case, we believe the h i p - k n e e implant is preferable, in m o s t situations, to one alternative, which is a total femoral replacement, owing to the preservation of muscle and soft tissue attachments.

Combined Hip-Knee Replacements

Summary A c o m b i n e d h i p - k n e e i m p l a n t c a n b e d e s i g n e d to h a v e sufficient s t r e n g t h . T h e surgical p r o c e d u r e is feasible, a n d t h e p o s s i b i l i t y of f u n c t i o n a l r e s t o r a t i o n h a s b e e n d e m o n s t r a t e d . It is s u g g e s t e d t h a t t h e h i p - k n e e i m p l a n t is a u s e f u l t r e a t m e n t for t h e serious problems described.

Acknowledgments W e a c k n o w l e d g e Ms. S. S a t h a s i v a m for a s s i s t a n c e r e g a r d i n g t h e a n a l y t i c a l p r o c e d u r e s , a n d Ms. C. C o a k l e y for c o l l a t i o n of t h e m e d i c a l r e c o r d s .

References 1. Inglis AE, Walker PS: Revision of failed knee replacements using fixed axis hinges. J Bone Joint Surg Br 73:757, 1991 2. Bethea JS, DeAndrade JR, Fleming EL, et al: Proximal femoral fractures following total hip arthroplasty. Clin Orthop 170:95, 1982

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3. Jensen JS, Barford G, Hansen D, et al: Femoral shaft fracture after hip arthroplasty. Acta Orthop Scand 59:9, 1988 4. Duda GN, Schneider E, Chao EYS: Internal forces and moments in the femur during walking. J Biomech 30:933, 1997 5. Taylor SJ, Walker PS, Perry J, et al: The forces in the distal femur and knee during different activities measured by telemetry. Trans Orthop Res Soc 22:259, 1997 6. Taylor SJ, Walker PS, Perry J, et al: The forces in the distal femur and the knee during walking and other activities measured by telemetry. J Arthroplasty 13: 428, 1998 7. Krause W, Mathis RS: Fatigue properties of acrylic bone cements: review of the literature. J Biomed Mater Res 22(suppl A1 ):37, 1988 8. Lee AJC, Ling RSM, Vangata SS: Some clinically relevant variables affecting the mechanical behaviour of bone cement. Arch Orthop Traumat Surg 92:1, 1978 9. Saha S, Pal S, Albright JA: Time dependent mechanical properties of normal and carbon fiber reinforced bone cement. Trans Orthop Res Soc 6:296, 1981 10. Brochure. IMI Titanium Ltd, Birmingham, UK