Hard tissue replacement (HTR) polymer as an implant material M.H. Amler* and R. Z. LeGeros Department of Oral Medicine and Pathology and Department of Dental Material Sciences, New York University CoZlege of Dentistry, 345 East 24th Street, New York, Nau York 10010 This study aimed to evaluate the effectiveness of a synthetic implant material Hard Tissue Replacement polymer (HTR) for: (1)compatibility with bone and soft tissues, (2) capacity to physically attach to bone and soft tissues, and (3) capacity for bone induction and metaplasia. HTR was implanted for a 3-week test period in femur bones, connective tissue, and skeletal muscle of 15 Sprague-Dawley descent rats for histological examination and implanted in bone in 6 rats for infrared absorption a n a l y s e s to d e t e r m i n e t h e presence of new bone. Compatibility (de-

fined as absence of significant inflammation) was present in 13/14 (93%)bone sites, 7/9 (78%) connective tissue, and 4/4 (100%)muscle sites. Physical attachment of HTR occurred in 10/14 (71%) bone sites, 4/9 (44%) connective tissue, and 1/7 (14%) muscle sites. Density of new bone appeared to be greater with HTR than in controls. However, no metaplastic bone was formed in nonbony sites indicating that this material is nonosteogenic. These preliminary findings demonstrated the effectiveness of HTR as an implant material.


Clinical and experimental studies with Hard Tissue Replacement (HTR) polymer have indicated the feasibility of using it as an implant material. As early as the 1970s positive .results with HTR were reported following implantation in both skeletal and nonskeletal sites.',2 Ashman et al.3-5indicated successful ridge augmentation for prosthetic reconstruction. Most recently, B i n d e ~ m a n suggested ~,~ the implantation of HTR impregnated with calcium hydroxide for treatment of long bone defects to prevent fibrous union. Until recently most evaluation trials of HTR were anecdotal reports or unstructured studies. This experimental investigation was intended to evaluate, more systematically, the effectiveness of HTR as an implant material. Three critical properties were examined histologically: (1) compatibility with bone and soft tissues, (2) capacity to physically attach to bone and soft tissues, and (3) capacity for bone induction and metaplasia. Infrared absorption analyses was used to evaluate new bone formation. *To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 24, 1079-1089 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/081079-11$04.00



Hard Tissue Replacement, (HTR) polymer, is a synthetic, porous, plastic material made of granular, biconcave pellets of polymethylmethacrylate coated with hydroxyethylmethacrylate and impregnated with calcium hydroxide. The resultant material is hydrophilic and structurally sound with an in vitro compression strength of 800-1,000 psi.' A particle size of 800 pm was utilized in this study. Twenty-four male, Sprague-Dawley descent rats, average weight of 400 g each, were utilized. A 2-mm trephine was used to create two openings in each right femur at opposite ends of the diaphysis for controls. The proximal end was left void and the distal end was implanted with autogenous bone matrix taken from the parietal bone. A single opening was created in the left femur and implanted with HTR. Adjacent to this site HTR was also placed in the outer layer of the periosteum. The right biceps femoris muscle was implanted with autogenous bone and the left biceps femoris with HTR. Animals were sacrificed after a 3-week test period. All tissues were fixed in 10% neutroformalin, bone specimens were decalcified, tissues were paraffin blocked,' sectioned utilizing a tungsten-carbide blade (180 mm TungstenCarbide Blade Reichert-Jung Inc., Buffalo, NY) for routine histologic examination. The tissues were compared according to compatibility with host tissues (defined as absence of significant inflammation) and physical attachment to host tissues. Three animals were not used for analysis because of fracture of the femur during the test period, leaving a total of 21 rats. Bony tissues from 6 rats were utilized for infrared absorption studies to characterize the material formed around the HTR beads after implantation. The remaining 15 rats each yielded 6 study tissues: 3 bone, 1connective tissue, and 2 skeletal muscle. RESULTS

Compatibility Compatibility is defined as absence of significant inflammation, and was 100% in skeletal muscle, 93% bone, and 78% connective tissue (Table I, Figs. 2,6).

Physical attachment In Figures 1 and 2 portions of the HTR beads can be observed physically attached to bone, as evidence that they were cut by the microtome blade and not entirely avulsed in sectioning. Attachment of HTR was present in 10/14 (71%) in bony sites and 4/9 (44%) in connective tissue (Fig. 3). Figure 4 represents the result of an experimental void in the femur. Newly




’Absence of significant inflammation.


Bone Muscle 13

9 7

Autogenous bone Autogenous bone


Number of Specimens


Implant Site

Bone Connective tissue Muscle


Implant Material


92 100


54 100




14 100

79 100 71 44



(”/.I 93 78 100




TABLE I Critical Properties of Hard Tissue Replacement (HTR) Polymer as an Implant Material after Three-Week Test Period









3a 3



Figure 1. HTR implanted into the marrow of a femur bone for the 3-week test period. HTR is well integrated in bony tissue with part of the HTR cut by the microtome blade (H&E, original magnification ~40).

regenerated bone attachment in the area where this void was created was 15/15 (100%). For the autogenous bone control in the femur, attachment was 7/13 (54%), Table I, Fig. 5). In muscle implantation of HTR there was evidence of physical attachment of HTR in 1/7 (14%)of cases; in all other cases pellets were avulsed by the microtome blade (Fig. 6). Figure 7 demonstrates a section of autogenous bone implanted into skeletal muscle. Attachment was 4/4 (100%). Bone induction/metaplasia

No evidence of bone induction or metaplastic bone formation was observed in any of the sections. DISCUSSION

Of 90 study tissue specimens that were recovered, 29 were technically unsatisfactory because of difficulties in processing before the tungsten-



Figure 2. A higher magnification of Figure 1. Biconcave form of pellet of HTR is apparent. No significant inflammation can be discerned. However, some capsule formation is apparent in upper right corner of area where a portion of the HTR has been cut away by the microtome blade. Macrophages (perhaps osteoclasts) are also noted on the periphery of the implant (H&E, original magnification x 125).

carbide microtome blade was obtained. As a result the available number of specimens of various implants and controls varied. In this study the striking aspect of HTR material was its high degree of compatibility with the host tissues. Bone growth that protruded into the core of this pellet did not result from the resorption of the pellet but resulted from the biconcave form of the pellet itself. Since the HTR compression strength was approximately 800-1,000 psi,' it follows that when the pellets were cut, the strength of their attachment to the host tissue must have been > 800 psi. The physical attachment to host tissues was demonstrated by the high proportion of HTR pellets in bone that were cut by the microtome blade rather than being avulsed (71%) (Figs. 1,2). Figures 1 and 2 also indicate significant regeneration of bone following the implantation of HTR. However, in the control implantation of autogenous bone as indicated in Figure 5 and in an additional control where a void was created and nothing implanted (Fig. 4) sigruficant bone regeneration was also observed. While it was beyond the scope of this study to precisely



Figure 3. HTR is implanted in periosteal connective tissue on periphery of femur bone, The HTR pellet has been sectioned rather than avulsed while to the left a void indicates where a second HTR pellet has been completely avulsed (H&E, original magnification X40).

characterize the comparative histological development of bone regeneration following implantation of HTR vs. the two controls there appeared to be a higher density of bone regeneration in the specimens where HTR had been implanted (Figs. 1,2) in direct comparison to the controls (Figs. 4,s).No relationship was found between encapsulation of implants and physical attachment in either the HTR polymer implants or in the controls. Infrared analysis of the bone implants showed the formation of apatitic material similar to bone mineral and to materials associated with other cal-



Figure 4. Tissue section where a void has been produced in the femur bone as a control. Normal spicule regeneration is observed (H&E, original magnification X 40).

cium phosphate ceramics (hydroxyapatite, HA and biphasic calcium phosHowever, the HTR material, phates, HA + b-TCP) after like other materials tested in this laboratory,” was nonosteogenic. It can be assumed that the new bone formation following implantation of HTR was the result of a physiologic process rather than reaction to an irritant since no significant inflammatory reaction was apparent. In view of the high level of compatibility, physical attachment to the host tissues and bone regeneration, these preliminary studies demonstrated the effectiveness of HTR as an implant material.



Figure 5. Autogenous bone implant in femur bone for the 3-week test period. No significant inflammatory reaction is apparent. Bone implant appears to be well tolerated by the host bone with a physical connection to host tissue (H&E, original magnification x40).


Figure 6. Implantation of HTR beads into skeletal muscle. No physical attachment to muscle can be noted since HTR has been completely avulsed by the microtome blade in this particular test animal. A well-defined endomysium encompasses the periphery of the area where the HTR was implanted. No significant inflammatory reaction is observed, although macrophages (osteoclasts?)are apparent (H&E, original magnification x 125).




Figure 7. Autogenous bone implanted in skeletal muscle as a control for the 3-week test period. Endomysium encapsulization is observed. Complete physical attachment is noted and no significant inflammatory reaction is apparent (H&E, original magnification ~40).

References 1. A. Ashman and M. L. Moss, “Implantation of porous polymethylmethacrylate resin for tooth and bone replacement,” J. Prosthef: Dent., 37, 657-664 (1977). 2. R.A. Abrahams, A. Ashman, and P. Bruins, “Technique for the rapid fabrication of customized hard tissue replacement (HTR),” Trans. Am. Soc. Artif. Intern. Organs, 28, 469472 (1982). 3. A. Ashman and I? Bruins, “HTR (hard tissue replacement) for edentulous ridge augmentation,“ N.Y. J. Dent., 53, 387-392 (1983). 4. A. Ashman and P. Bruins, ”Prevention of alveolar bone loss postextraction with grafting material,” Oral. Surg. Oral. Med. Oral Puthol., 60, 146-153 (1985). 5. A. Ashman, S. E. Neuwirth, and P. Bruins, ”The HTR molded ridge for alveolar augmentation -an alternative to the subperiosteal implant, autogenous bone graft or injectable bone grafting materials,” J. Oral lmplantol., 12, 556-575 (1986). 6. I. Binderman, M. Goldstein, I. Horowitz, N. Fine, S. Taicher, A. Ashman and A. Shteyer, “Comparison of bone grafting materials in experimental long bone gaps in rats,” J. Orthoped Surg., in press. 7. I. Binderman, M. Goldstein, I. Horowitz, N. Fine, S. Taicher, A. Ashman, and A. Shteyer, Graftsof HTR Polymer versus Kiel Bone in Experimental Long Bone Defects in Ruts, American Society for Testing and Materials, Philadelphia, 1988, pp. 370-376.



8. A. Schulman, Personal communication, 1989. 9. W. T. Dempster, "Paraffin compression due to the rotary microtome," Stain Tech, 18, 13-25 (1943). 10. R. Z. LeGeros, J. P. LeGeros, 0.R. Trautz, and E. Klein, "Spectral properties of C 0 3in C0,-containing apatites," Ada Spec. 7B, 3-12 (1970). 11. M. Heughebaert, R. Z . LeGeros, M. Gineste, A. Guilhem, and B.

Bonel, "Physicochemical characterization of deposits associated with HA ceramics implanted in non-osseous sites," 1. Biomed. Mafer. Res., 22, 257-268 (1968). 12. G. Daculsi and R. Z. LeGeros, "Transformation of biphasic calcium phosphate ceramics in vivo," J. Biomed. Muter. Res., 23, 883-894 (1989). 13. M. H. Amler, "Osteogenic potential of non-vital tissues and synthetic implant materials," J. Periodontol., 58, 758-761 (1988). Received April 18, 1989 Accepted February 23, 1990

Hard tissue replacement (HTR) polymer as an implant material.

This study aimed to evaluate the effectiveness of a synthetic implant material Hard Tissue Replacement polymer (HTR) for: (1) compatibility with bone ...
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