Impacted corticocancellous allografts: Recoil and strength

The Journal of Arthroplasty Vol. 14 No. 8 1999

Impacted Corticocancellous Allografts Recoil and Strength G6sta Ullmark,

MD,* and

Olle Nilsson,

MD~-

Abstract" Bone allografts were morcellized using 2 different milling machines (Tracer milling machine and Howex milling machine) producing bone chips with different ranges of sizes. In an in vitro model, each type of bone-graft was impacted with 2 different impaction forces. As the impaction force was released, there was a substantial and rapid recoil of the graft bed. More recoil was seen with the smaller chips and with the harder impacted graft beds. Most of the recoil occurred within the first 10 seconds. Using another similar in vitro model, the same 2 kinds of chips were impacted with 2 different impaction forces, and the subsidence of the 4 different graft beds was measured. There was less subsidence with the bigger type of bone chips and less with the harder impacted graft beds. K e y words: biomechanical properties, graft impaction, revision.

Bone defects are frequently f o u n d at revision arthroplasty, especially of the hip or knee, and m a y pose a m a j o r challenge to the surgeon. The surgical objectives are to obtain l o n g - t e r m and s h o r t - t e r m stability of the implant and reconstitution of the bone. The use of impacted cancellous allograft and c e m e n t at revision hip arthroplasty with m a j o r b o n e defects has b e e n reported to give good results b o t h for the a c e t a b u l u m [1,2] and for several different femoral implants, including collarless polished doubletapered Exeter s t e m ( H o w m e d i c a , R u t h e r f o r d , N J) [3], collarless polished taper Z i m m e r stem (Zimmer, Warsaw, IN) [4], Lubinus SP II m a t t stem with a collar (Waldemar Link, Hamburg, G e r m a n y ) , and m a t t C h a r n l e y Elite plus stem (DePuy, Leeds, UK) [5]. Impaction cancellous allografting with

c e m e n t at revision k n e e arthroplasty has also b e e n s h o w n to give good results [6,7]. The surgical procedure of impaction grafting entails impaction of the morcellized allograft in a contained b o n e defect. The graft should be impacted so that the forces on the prosthesis after surgery do not cause further impaction of the graft and subsidence of the prosthetic c o m p o n e n t s . To ensure stability of the implant, sufficient pressure m u s t be m a i n t a i n e d on the graft bed during extrusion of the impaction p h a n t o m , cementing, and introduction of the prosthetic c o m p o n e n t , until the c e m e n t has cured. One prerequisite for maintaining this pressure is c o n t a i n m e n t of the graft. Although the p r i m a r y stability of the implant is essential for the clinical outcome, little is k n o w n of the mechanical p e r f o r m a n c e of impacted corticocancellous allografts before ingrowth of living tissue. We studied properties of the b o n e - g r a f t that m a y be of special i m p o r t a n c e for the early stability of the implant. Morcellized and impacted b o n e - g r a f t recoils w h e n the impaction i n s t r u m e n t is r e m o v e d . We studied the m a g n i t u d e of recoil in the graft bed and the speed of recoil. We further investigated the effects of the initial impaction force and the size of the impacted b o n e chips on the recoil. The surface

From lhe *Orthopaedic Department, L~nssjukhuset, G~vle; and tOrthapaedic Department, Uppsala Akademiska sjukhus, Uppsala, Sweden. S u b m i t t e d J u n e 12, 1998; accepted F e b r u a r y 23, 1999. Funds were received from the Swedish Medical Research Council (B94-17X-06577-12A) in support of the research m a t e rial described in this article. Reprint requests: GOsla Ullnlark, MD, Orthopaedic Departm e n t , L~inssjukhuset, 801 87 G~vle, Sweden. Copyright © 1999 by Churchill Livingstone ~ 0 8 8 3 - $ 4 0 3 / 9 9 / 1 4 0 8 - 0 0 2 1 $10.00/0

1019

1020

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

r e s i s t a n c e to l o a d of b o n e chips f r o m 2 t y p e s of m i l l i n g m a c h i n e s a n d w i t h d i f f e r e n t initial i m p a c t i o n forces w a s also s t u d i e d .

Material and Methods T w o in vitro m o d e l s w e r e used, t h e first to d e m o n strate t h e recoil of a graft b e d w h e n t h e i m p a c t i o n force is r e l e a s e d ( e x p e r i m e n t 1) a n d t h e s e c o n d to i n v e s t i g a t e t h e l o a d r e s i s t a n c e of a n i m p a c t e d graft b e d ( e x p e r i m e n t 2). Fig. 1. Model for impaction of morcellized allograft.

Experiment 1 A l l o g e n e i c b o n e w a s t a k e n f r o m h u m a n fresh f r o z e n f e m o r a l h e a d s o b t a i n e d at p r i m a r y h i p j o i n t r e p l a c e m e n t for c o x a r t h r o s i s a n d s t o r e d in a f r e e z e r at - 8 0 ° C . The b o n e s w e r e c l e a n s e d f r o m soft tissues. Two d i f f e r e n t b o n e mills w e r e u s e d . The H o w e x b o n e mill ( H o w e x , G/ivle, S w e d e n ) p r o d u c e s s o m e w h a t b i g g e r chips w i t h a v o l u m e r a n g e of 0.0002 to 40 m m ~ (0.05 × 0.06 × 0.08 m m to 2 × 4 × 5 r a m ) t h a n t h e Tracer b o n e mill (Tracer, St Paolo, CA), w i t h a r a n g e of 0 . 0 0 0 2 to 12 m m ~ (0.03 × 0.07 X 0 . 0 8 m m to 1 × 3 × 4 m m ) , as m e a s u r e d in a light m i c r o s c o p e after t h e b o n e chips w e r e i m p a c t e d , r e f r a g m e n t e d , a n d d r i e d in air. M o r c e l l ized chips w e r e d e f a t t e d b e f o r e i m p a c t i o n b y p l a c i n g t h e m in a t o w e l a n d r e p e a t e d l y w a s h i n g w i t h a w a r m saline s o l u t i o n ( 4 0 ° - 5 0 ° C ) . The i m p a c t i o n p r e s s u r e s w e r e c h o s e n to m i m i c t h e clinical s i t u a t i o n so t h a t t h e graft b e d s h a d a s i m i l a r stability as in t h e clinical setting, a c c o r d i n g to o u r e x p e r i e n c e s f r o m 100 h i p i m p a c t i o n graftings. A n i m p a c t i o n p r e s s u r e of 55 N / c m 2 ( H o w e x , n -- 8; Tracer, n = 5) w a s c o m p a r e d w i t h I 9 5 N / c m 2 ( H o w e x , n = 6; Tracer, n = 5). A v o l u m e w i t h 20 c m 2 of surface l i m i t e d b y 4 f r a m e s of 20 m m in h e i g h t w a s u s e d . T h e t h i c k n e s s e s of t h e graft b e d s w e r e 9.8 to 15.0 m m . T h e b o n e - g r a f t w a s i m p a c t e d b y p r e s s u r e in a r e c t a n g u l a r - s h a p e d f o r m u s i n g a m e t a l lid (Fig. 1 ). P r e s s u r e w a s a p p l i e d b y s c r e w i n g a bolt with a calibrated torque wrench. The threaded bolt (M12) w a s l u b r i c a t e d w i t h oil, a n d t h e friction coefficient w a s 0.25. T h e level of t h e m e t a l lid in r e l a t i o n to t h e f r a m e s w a s m e a s u r e d o n 2 d i a g o n a l c o r n e r s b y callipers ( a c c u r a c y 0.05 m m ) . The graft b e d s w e r e h e l d u n d e r p r e s s u r e for 45 _+ 5 s e c o n d s . As t h e i m p a c t i o n p r e s s u r e w a s r e l e a s e d , t h e recoil w a s m e a s u r e d a f t e r 10, 30, a n d 60 s e c o n d s . T h e t i m e for e a c h m e a s u r e m e n t w a s 3 to 4 s e c o n d s . The recoil w a s c a l c u l a t e d as a p r o p o r t i o n of t h e initial t h i c k n e s s of t h e graft b e d .

Experiment 2 T h e s a m e 2 t y p e s of a l l o g e n e i c h u m a n b o n e chips f r o m t h e H o w e x m i l l i n g m a c h i n e a n d t h e Tracer m i l l i n g m a c h i n e , d e f a t t e d in t h e s a m e m a n n e r a n d i m p a c t e d w i t h t h e s a m e i n s t r u m e n t s in t h e s a m e m o d e l as in e x p e r i m e n t 1, w e r e u s e d . The i m p a c t i o n p r e s s u r e s for b o t h t y p e s of chips w e r e 62 N / c m 2 ( H o w e x , n = 6; Tracer, n = 5) or 195 N / c m 2 ( H o w e x , n = 5; Tracer, n = 6). The t h i c k n e s s e s of t h e graft b e d s w e r e 11.1 to 15.6 m m . A f t e r r e l e a s i n g t h e i m p a c t i o n force, a s m a l l e r a r e a of I c m 2 w a s l o a d e d in t h e c e n t e r of t h e o p e n 2 0 - c m 2 graft b e d (Fig. 2). S u b s i d e n c e of t h e 1 - c m 2 p l a t e in t h e graft b e d w a s m e a s u r e d at 4 l o a d i n g p r e s s u r e s : 154, 309, 617, a n d 1,230 N / c m 2. The p r e s s u r e s w e r e c h o s e n to m i m i c t h e p h y s i o l o g i c l o a d s o n t h e h i p j o i n t . T h e subsid e n c e w a s m e a s u r e d at t h e h e a d of t h e b o l t a d a p t e d to t h e 1-cm 2 p l a t e w i t h t h e s a m e callipers. Statistical calculations were performed on the original meas u r e m e n t d a t a u s i n g f a c t o r i a l a n a l y s i s of v a r i a n c e .

! I

t ~,,~ ~

~J,~~A." .',4r'~D,'~,

Fig. 2. Model for measurement of subsidence of a loaded 1-cm 2 plate on an open 20-cm 2 graft bed.

Impacted CorticocancellousAIIografts

Results The recoil was found to be significantly higher for the smaller (Tracer) bone chips compared with the larger (Howex) bone chips (P < .001). There was also a significantly larger recoil at the higher impaction force for both types of chips (P < .001) (Fig. 3). The subsidence was found to be significantly higher for the graft beds impacted with less force (62 N/cm 2) (P < .001). There was also a significantly larger subsidence caused by increasing loads (P < .001) as well as for the smaller (Tracer) bone chips (P < .001) (Fig. 4). The proportion of recoil did not differ because of the thickness of the graft bed. Discussion

The present study suggests larger chip sizes are preferable for the p r e v e n t i o n of subsidence and recoil. It also suggests a high degree of impaction of the graft bed is favorable for the prevention of subsidence and other m o v e m e n t s of the prosthetic c o m p o n e n t s after surgery. Subsidence of the prosthetic c o m p o n e n t s in revision surgery with impaction bone grafting might be associated with increased risk for short-term and long-term complications. The present investigation was performed to study the effect of the different degrees of graft impaction and size of the bone chips on the mechanical properties of the graft. In this experimental model, a substantial recoil of the impacted graft bed ( 1 1 % - 3 4 % ) was measured. Most of the recoil took place in the first few seconds after releasing the impaction force. The recoil was significantly higher using smaller bone chips. There

•

UIImarkand Nilsson

1021

was also a significantly higher recoil in impacted graft beds with a higher force. According to Giesen et al. [8], an impacted morcellized graft has the mechanical characteristics of an elasto-viscoplastic material. In an vitro test on a h u m a n cadaver femur with impacted morcellized cancellous allograft and cemented CPT Zimmer prosthesis [9], loaded with 2 times the body weight for 5,000 cycles, a subsidence of the femur compon e n t of 0.25 m m more than in the same model without impacted allograft was observed. Most of the subsidence took place during the first cycles. Morcellized bone allografts from h u m a n femoral heads consist of small pieces of cortical and trabecular bone. As pressure was applied on those chips, deformation of the trabecular bone probably occurred, which was, to a certain extent, reversible. Morcellized bone-graft also consists of small pieces of cortical bone lacking potential for deformation at the pressures used. As the morcellized bone-graft was pressed together, the v o l u m e of the graft bed could be diminished in 2 ways: by decreasing the distance b e t w e e n individual chips and by decreasing the size of the trabecular chips through (reversible) deformation. In the latter mechanism, part of the pressure is retained within the graft bed after the impaction procedure unless the graft bed is allowed to expand. In the present study, graft beds were compressed through a relatively constant compression force by a threaded bolt. In vivo compression is performed by administering intermittent blows on an impaction device. The present test models are mimicking the in vivo situation in a contained cavitary defect, such as in the acetabulum or the proximal tibia. Inside the cortical tube of femur, graft impaction is performed

RECOIL

Fig. 3. Recoil of impacted graft bed calculated as increased proportion of the initial graft bed thickness as the impaction pressure was released. T refers to chips from Tracer bone mill and H from Howex bone mill; 195 and 55 were the impaction forces (N/cm 2) of the graft beds. The bars represent the ranges of values.

.......................

T 195

1.30

H 195 1.20

......

~ --~

T 55 H55

1.10

1.00

I 10

I 20

I 30

I

I

:

40

50

60

-'-

TIME (sec.)

1022

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

Fig. 4. Subsidence of the 1-cm 2 plate in the graft bed calculated as decreased proportion of the initial graft bed thickness. H refers to chips from Howex bone mill and T from Tracer bone mill; 195 and 62 were the impaction forces (N/cm 2) of the graft beds. The bars represent the ranges of values,

o.5o t

,,,,'*'"

T 62 H 62

0.60 T 195

o.7o

H 195

0.80 o.90

1,00

',

:

200

in a s o m e w h a t different m a n n e r . A double-tapered polished p h a n t o m is driven in the grafted femoral tube producing not only compression forces, but also some shear. The differences in application of the impaction force are of u n k n o w n i m p o r t a n c e for graft stability and recoil. There are no published data on the a m o u n t of compression force caused by the impaction p h a n t o m in an allograft b o n e bed. To obtain an experim e n t a l m o d e l to study the forces on a morcellized b o n e allograft, the pressures were applied in a controlled m a n n e r by a screw. The forces applied in this e x p e r i m e n t were chosen to produce a graft bed with similar characteristics to grafts obtained during revision surgery. The e x p e r i m e n t s were p e r f o r m e d at a t e m p e r a t u r e of 20°C. Bleeding into the graft bed in vivo, resulting in fibrin f o r m a t i o n in a blood clot, might s o m e w h a t decrease the recoil and could affect the viscoelastic properties of the graft. Thus, the stability of the graft bed is probably s o m e w h a t affected by the in vitro conditions. Morcellized b o n e - g r a f t impacted in a contained space achieves a certain stability, which depends on the a m o u n t of the initial impaction force, the size of the b o n e chips, and probably also the shape of the b o n e chips. Because the graft bed is loaded with a compression force, there is also a certain a m o u n t of shear. The shear strength is higher after an initially higher impaction force, with a bigger size of b o n e chips, and in grafts with a lower fat content [10]. In civil engineering and soil mechanics, particle size is s h o w n to have an optimal spread in terms of increased stability [I 1,12]. This spread of sizes also includes a fraction of larger-sized particles. After impaction bone grafting, patients h a v e usually been confined to bed for some weeks. Most

I

I

400

I

I

600

I

I

800

I

I

I

1000

1200

•

LOAD N/cm 2

authors suggest that patients should walk with t o e - t o u c h weight bearing on the operated leg for 6 to 12 weeks because of the risk for subsidence of the femoral implant and migration or even rotation of the acetabular implant [2,3,4,131. Subsidence m a y interfere with the b o n e - h e a l i n g mechanisms, alter the a l i g n m e n t of the prosthetic c o m p o n e n t s , and cause an early failure [14]. If the surgeon is hesitant to exert a hard blow to the impaction p h a n t o m because of the risk for fracture, prophylactic cerclage wiring should be p e r f o r m e d to facilitate for a hard graft impaction. If the design of the impaction p h a n t o m does not take the p h e n o m e n o n of recoil of the graft bed into consideration, the space for the c e m e n t m a n t l e produced m a y be too small because of recoil, resulting in an incomplete c e m e n t mantle. If the p h e n o m e n o n of recoil of the graft bed is not t a k e n into consideration during a revision procedure, the prosthetic c o m p o n e n t m a y be implanted too deep in relation to the impaction p h a n t o m , or an error m a y be m a d e in releasing the impaction p h a n t o m too long a time before c e m e n t introduction. It is necessary to m a i n t a i n a substantial compression on the c e m e n t and thus recompress the b o n e - g r a f t after the prosthetic c o m p o n e n t is inserted until the c e m e n t has cured. The space for the c e m e n t mantel produced from the impaction p h a n t o m m a y otherwise disappear because of recoil, resulting in an incomplete or even fragmental c e m e n t mantel. The graft bed might also be recompressed postoperatively by loading, resulting in an early subsidence. Slooff has r e c o m m e n d e d biggersized chips for the a c e t a b u l u m c o m p a r e d with the femur. The space for chips in b e t w e e n the endost i u m and the impaction p h a n t o m in the f e m u r is limited in a m o r e definite w a y as c o m p a r e d with the

Impacted CorticocancellousAIIografts a c e t a b u l u m . Thus, big chips can c r e a t e a n i n c o m p l e t e graft i m p a c t i o n in t h e f e m u r c o m p a r e d w i t h t h e a c e t a b u l u m . T h e q u e s t i o n w h e t h e r chips of a b i g g e r size t h a n t h o s e i n v e s t i g a t e d in t h e p r e s e n t s t u d y w o u l d be e v e n m o r e stable is u n k n o w n . W e h a v e n o t f o u n d s u c h a study.

References 1. Olivier H, Sanouiller JL: Acetabular construction using morcelised bone grafts in the revisions of total hip arthroplasties. Rev Chir Orthop 77:232, 1991 2. Slooff TJJH, Huiskes R, van Horn J, Lemmens AJ: Bone grafting in total hip replacement for acetabular protrusion. Acta Orthop Scand 55:593, 1984 3. Gie GAA, Linder L, Ling RSM, et al: Impacted cancellous allografts and cement revision total hip arthroplasly. J Bone Joint Surg Br 75:14, 1993 4. Elting J J, Mikhail WEM, Zicat WA, et al: Preliminary report of impaction grafting for exchange femoral arthroplasty. Clin Orthop 319:159, 1995 5. Ullmark G, Lundberg BJ, Josefsson G, et al: Impacted cortico cancellous allografts and cement for revision total hip arthroplasty using Lubinus and Charnley prosthesis. Acta Orthop Scan 68 (suppl 274):8, 1997 6. Ullmark G, Hovelius L: hnpacted morsellized allograft and cement for revisions total knee arthroplasty. Acta Orthop Scand 67:10, 1996

•

UIImarkand Nilsson

1023

7. de Waal Malefi]it MC, van Kampen A, Slooff TJJH: Bone grafting in cemented knee replacement: 45 primary and secondary cases followed for 2-5 years. Acta Orthop Scand 66:325, 1995 8. Giesen G, Weinans H, Lamerigts N, Huiskes R: Mechanical characteristics of impacted morsellized bone grafts used in revision total hip arthroplasties. Acta Orthop Scand 67 (supp1272):92, 1996 9. Malkani AL, Voor M J, Fee KA, Bates CS: Femoral component revision with impacted morsellized cancellous graft: Biomechanical study of implant stability. J Bone Joint Surg Br 78:973, 1996 10. Ullmark G: Subsidence within impacted cortico cancellous allografts: biomechanical studies. Acta Orthop Scand 68(suppl. 274):8, 1997 11. Marks VJ, Monroe RW, Adam JF: Effects of crushed particles in asphalt mixtures. Transportation Research Record 1259:91, 1990 12. Woodside AR, Lyle E Kelly B: Enhancing the shape characteristics of an aggregate to improve its pavement performance in the highway. Read at PTRC 19th annual meeting at the University of Sussex, England, Sept 9-13, 1991 13. Slooff TJJH, Schimmel JW, Buma P: Cemented fixation with bone grafts. Orthop Clin North Am 24:667, 1993 14. Eldridge JDJ, Smith EJ, Hubble MJ, et al: Massive early subsidence following femoral impaction grafting. J Arthroplasty 12:535, 1997