Impact Insertion of Osteochondral Grafts: Interference Fit and Central Graft Reduction Affect Biomechanics and Cartilage Damage

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1 Impact Inserton of Osteochondral Grafts: Interference Ft and Central Graft Reducton Affect Bomechancs and Cartlage Damage Alvn W. Su, 1,2 Yunchan Chen, 1 Dustn H. Wales, 1 Van W. Wong, 1 Shengqang Ca, 2,3 Albert C. Chen, 1 Wllam D. Bugbee, 4,5 Robert L. Sah 1,2,6 1 Department of Boengneerng, Unversty of Calforna-San Dego, La Jolla, Calforna, 2 Program n Materals Scence and Engneerng, Unversty of Calforna-San Dego, La Jolla, Calforna, 3 Department of Mechancal and Aerospace Engneerng, Unversty of Calforna-San Dego, La Jolla, Calforna, 4 Center for Musculoskeletal Research, Insttute of Engneerng n Medcne, Unversty of Calforna-San Dego, La Jolla, Calforna, 5 Department of Orthopaedc Surgery, Scrpps Clnc, La Jolla, Calforna, 6 Department of Orthopaedc Surgery, Unversty of Calforna-San Dego, La Jolla, Calforna Receved 7 March 2017; accepted 13 June 2017 Publshed onlne 6 July 2017 n Wley Onlne Lbrary (wleyonlnelbrary.com). DOI /jor ABSTRACT: An osteochondral graft (OCG) s an effectve treatment for artcular cartlage and osteochondral defects. Impact of an OCG durng nserton nto the osteochondral recpent ste (OCR) can cause chondrocyte death and matrx damage. The am of the present study was to analyze the effects of graft-host nterference ft and a modfed OCG geometry on OCG nserton bomechancs and cartlage damage. The effects of nterference ft (radus of OCG - radus of OCR), loose (0.00 mm), moderate (0.05 mm), tght (0.10 mm), and of a tght ft wth OCG geometry modfcaton (central regon of decreased radus), were analyzed for OCG cylnders and OCR blocks from adult bovne knee jonts wth an nstrumented drop tower apparatus. An ncreasngly tght (OCG - OCR) nterference ft led to ncreased taps for nserton, peak axal force, graft cartlage axal compresson, cumulatve and total energy delvery to cartlage, lower tme of peak axal force, lesser graft advancement durng each tap, hgher total crack length n the cartlage surface, and lower chondrocyte vablty. The modfed OCG, wth reducton of dameter n the central area, altered the bomechancal nserton varables and bologcal consequences to be smlar to those of the moderate nterference ft scenaro. Mcro-computed tomography confrmed structural nterference between the OCR bone and both the proxmal and dstal bone segments of the OCGs, wth the central regons beng slghtly separated for the modfed OCGs. These results clarfy OCG nserton bomechancs and mechanobology, and ntroduce a smple modfcaton of OCGs that facltates nserton wth reduced energy whle mantanng a structural nterference ft Orthopaedc Research Socety. Publshed by Wley Perodcals, Inc. J Orthop Res 36: , Keywords: artcular cartlage; cartlage mechancs; mpact mechancs; osteochondral autograft; osteochondral allograft Repar wth osteochondral graft (OCG) s one of the most effectve surgcal treatments for focal artcular cartlage defects. 1,2 However, durng nserton, mpact loads are appled to the artcular cartlage (AC) of the OCG and may result n chondrocyte death. 3 Vable chondrocytes and cartlage matrx ntegrty are mportant for clncal success of the surgery. 4,5 Typcally, such OCG are prepared to be cylndrcal, as are the osteochondral recpent (OCR) stes. The bomechancs durng the OCG nserton process reflect a seres of mpact events. The surgeon apples taps (N total) to nsert the OCG nto the OCR. For each tap (), the peak force (F p ) ncreases as the OCG advances deeper. 6,7 F p ncreases substantally as the OCG s seated and bottoms out, 6,8 partcularly for shorter OCGs that have relatvely hgh structural stffness. 9 The duraton of each tap (T ) s bref, rangng from 0.5 to 5 ms. 3,5,7,8 However, the dstrbuton of the mpact energy delvered to the graft, as well as the resultant cartlage compressve stran and extent of OCG advancement, have not been reported. Understandng the mechanobology of AC damage s mportant for preservng graft tssue health durng [Correcton added on February 15, 2018, after frst onlne publcaton: author name Wong W. Van corrected to Van W. Wong] Grant sponsor: Natonal Insttutes of Health; Grant numbers: R01 AR055637, R01 AR044058, P01 AG007996; Grant sponsor: Natonal Yang-Mng Unversty; Grant number: n/a. Correspondence to: Robert L. Sah, (T: ; F: ; E-mal: rsah@ucsd.edu) # 2017 Orthopaedc Research Socety. Publshed by Wley Perodcals, Inc. surgery. Damage to the AC durng OCG mpact has been related to a varety of bomechancal factors. Durng OCG nserton, chondrocyte death has been assocated wth F p. 6,8 For non-nsertonal mpact scenaros, chondrocyte death has been assocated wth mpact force, 8 contact stress, 10,11 compressve stress rate, 12,13 compressve stran, 10,14 compressve stran rate, 15 and total mpact energy Artcular cartlage falure has been descrbed by fracture energy, or the energy normalzed to crack area, 19 so that the parameter of energy nput, or energy normalzed to cartlage area, may provde an ndex related to cartlage damage. The graft-host nterference ft (radus of OCG - radus of OCR, DR) between OCG and OCR s one of the key parameters for surgcal nstrument desgn. Studes of analogous stuatons have assessed the effect of DR on nserton mechancs. Analogously, when nals are drven nto wood, F p ncreases wth nal dameter. 20 Whle a relatvely tght ft can be benefcal to graft-host healng due to ncreased postnsertonal graft stablty, such a tght ft requres relatvely hgh mpact to acheve the nserton. The effect of DR on OCG nserton bomechancs and resultant cartlage damage remans to be elucdated. The hypotheses of the present study were that durng OCG nserton nto OCR, () ncreasng tghtness of graft-host nterference ft leads to hgher nserton energy and resultant AC damage, and () a modfed OCG geometry alters the mechancs of mpact nserton and therefore reduces nserton energy and resultant AC damage. The specfc ams were to 377

2 378 SU ET AL. determne the effects of DR for standard cylndrcal OCG and OCR on () nserton bomechancs and () damage to AC cells and tssue, and also () to compare standard OCG and modfed OCG for nserton bomechancs and the resultant AC damage. MATERIALS AND METHODS Study Desgn Study 1 The effects of DR on bomechancs of nserton and damage to artcular cartlage were assessed n terms of the total number of taps (N), cumulatve energy densty delvered to cartlage of OCG (SW S, OCG ), and vablty of chondrocytes at the artcular surface (V AC,OCG ), as well as the relatonshp between cumulatve OCG advancement (u adv [m]) and energy delvered (W Tamp [m]) after the m th tap for standard cylndrcal OCG and OCR wth three study groups: () loose ft, DR ¼ 0.00 mm (n ¼ 7), () moderate ft, DR ¼ 0.05 mm (n ¼ 6), and () tght ft, DR ¼ 0.10 mm (n ¼ 6). Study 2 The effect of OCG geometry on nserton bomechancs and artcular cartlage damage was assessed by comparng the last group n Study 1 to an addtonal group, also wth orgnal DR ¼ 0.10 mm, but wth modfed OCG geometry as descrbed below to have a reduced DR n a central regon (n ¼ 7). Detaled Expermental Methods Sample Preparaton A total of 41 OCGs and 26 OCRs were prepared from the dstal femora of sx adult bovne knees, obtaned fresh from an abattor wthn 24 h of sacrfce and usng sterle technque. Wth a corng bt and damond-edged saw, OCGs were prepared to a radus of 2.40 mm and to a subchondral bone heght of 5.00 mm. The OCRs were prepared as osteochondral cubod blocks wth a base of mm 2, and heght of 15 mm. Of the 41 OCGs, 15 served as control samples wthout any mpact loadng treatment. In Study 1, 19 OCGs were used. In Study 2, seven OCGs were prepared to a modfed geometry. Durng OCG and OCR preparaton, all samples were kept most and cool by rrgaton wth phosphate buffered salne (PBS). Graft-Host Interference Ft (DR) Graft-host nterference ft (DR) was calculated as the radus of the OCG (2.40 mm) mnus the radus of the OCR (a OCR ). The recpent stes were drlled n the center of the cubod blocks to a depth of 10 mm from the artcular surface usng stanless steel drll bts of 4.80, 4.70, and 4.60 mm n dameter. Therefore, a OCR values were 2.40, 2.35, and 2.30 mm, respectvely, and the resultng three levels of DR were 0.00, 0.05, and 0.10 mm (Fg. 1A and C). OCG Wth Modfed Geometry Startng from the standard OCG, a central regon wth decreased radus was created. Specfcally, a 0.25 mm reducton n radus was created n a 2 mm long (deep) regon, extendng down from 1.5 mm below the cartlage-bone juncton (Fg. 1B). Ths was acheved by holdng the OCG n a collet (dameter 4.8 mm) of a mll machne, and then gradually translatng a 2 mm-thck rectangular hand fle, mounted on the platform of the drll mll, for 0.25 mm after ntal contact. Durng ths process, the sample was rrgated Fgure 1. Schematc of geometres of OCG and OCR. (A) standard OCG, (B) modfed OCG, and (C) OCR. Indcated s graft-host nterference ft (DR). wth PBS to cool and hydrate the tssue. In addton, care was taken to not dsturb the artcular surface. OCG Inserton Standard and modfed OCG were nserted n a standardzed way, applyng a prescrbed energy densty, W S, PE, for each tap and recordng the resultant mpact load, F (t), the movement of the cartlage surface, u Tamp (t), and the OCG advancement dstance wth each tap, u adv. The OCG was pre-nserted manually, past the AC, seatng t at the bone, 1.5 mm nto the OCR. A rgd tamp was then placed atop the AC surface. Seral taps were appled to the OCG va the rgd tamp, usng a drop tower apparatus nstrumented wth a load sensor at the mpacton surface, provdng F (t), and a laser dsplacement sensor assessng the poston of the artcular surface of the OCG va the tamp, provdng u Tamp (t). After each tap, a sde-vew dgtal photograph (0.02 mm pxel resoluton) was taken to allow measurement of the cumulatve OCG advancement dstance after tap, u adv [m], and from these, the ncremental OCG advancement dstance, u adv, for each tap. The appled energy densty, energy/ocg cartlage area, of each tap, W S, PE, was set by usng a weght at varous heghts (wth potental energy that s the product of mass, acceleraton due to gravty, and heght) from the OCG bone surface. The frst tap had W S,1 PE

3 OSTEOCHONDRAL GRAFT INSERTION INTERFERENCE FIT 379 of 0.9 mj/mm 2. Successve taps had energy densty ncreasng by a factor of 1.5, so that W S, PE ¼ ( 1) mj/mm 2. Successve taps were appled untl tap N, after whch the artcular surface of the OCG was flush wth that of the OCR (Fg. 2A). Chondrocyte Vablty Analyss After the fnal mpact, the artcular cartlage was analyzed for percentage of vable chondrocytes. 3,21 The artcular cartlage of the OCG was slced off as a dsc wth a sterle scalpel and ncubated for 24 h n medum wth 10% FBS. The dsc was then staned wth LIVE/DEAD 1 (Thermo Fsher Scentfc, Waltham, MA) as prevously descrbed, 21 maged en face wth fluorescence mcroscopy n the central ( mm) area, spannng most of the sample. Image processng was performed to determne vablty, V AC,OCG, as lve cells / (lve cells þ dead cells). Artcular Cartlage Surface Analyss Fnally, matrx damage was assessed by determnng L crack,the total length of cracks at the artcular surface. Samples were staned wth Inda Ink and photographed. 22 Usng NIH ImageJ software, both edges of each crack were traced, and the total edge lengths were summed. L crack was taken as half of the total edge length. The total surface crack length of each OCG was determned as the average of measurements by three ndependent observers. The measurement was hghly reproducble, wth ntra-observer correlaton coeffcent of Mcro-Computed Tomography (mct) Imagng One sample of each expermental group was chosen for mct scannng for qualtatve assessment of the top, mddle and bottom parts of graft-host subchondral bone nterface. The mct scan settngs were those descrbed prevously wth (9 mm) 3 voxel resoluton. 4 Bomechancs Calculatons All bomechancal varables and parameters are lsted n Table 1. Mechancal varables were quantfed from F p (t), u AC,OCG (t), and u adv. Peak contact stress (s p ) was calculated as the peak contact force (F p ) normalzed to the surface area of the OCG (A OCG, mm 2 ). Impact mpulse (I C ) was quantfed by ntegratng mpact force over tme for the duraton (T ) of the mpact. Peak axal dsplacement of AC of OCG (u AC,OCG p, )wastakenaspeak axal dsplacement of the tamp (u Tamp p, )mnustheocg advancement dstance durng for each tap (u adv ). Peak axal stran of the AC of OCG, e p, was calculated by normalzng u AC,OCG p, to the AC thckness of the OCG (h AC,OCG ). For each tap, the energy delvered by the tamp to the sample, W Tamp, was calculated by ntegratng F (t) over the axal dsplacement of the tamp, u Tamp (t). Energy delvered to the AC of the OCG, W OCG and to advancng the OCG nto OCR, W adv, were determned by an energy balance. For each tap,, W Tamp was assumed to be dvded nto W OCG and the work to advance the OCG nto the OCR, and accordngly apportoned to the cartlage by the peak deformaton of AC of OCG relatve to the movement of bone, wth W OCG ¼ W Tamp AC, (u p, OCG /u Tamp p, ), and W adv Tamp ¼ W (u adv /u Tamp p, ) (Fg. 2A). The correspondng energy densty delvered to the AC of an OCG by tap, W OCG S,, was calculated by normalzng W OCG to A OCG. The energy densty delvered to the bone nterface for OCG advancement nto OCR, W S, adv, was calculated by normalzng W adv to the ncrement n host-graft bone nterface area, A adv, encompassng the OCG travel dstance u adv, so that A adv ¼ p 4.80 mm u adv. Cumulatve energy delvered to OCG samples through tap m, W Tamp [m], was calculated as S m ¼1 WTamp. Cumulatve energy densty delvered to the artcular Fgure 2. Schematc of mpact nserton of OCG as well as mechancal varables and parameters. (A) Advancement of OCG nto OCR, from startng poston (A1), advancng wth successve taps (A2 5), to fnal flush poston (A5). Tap #,, out of N total taps. Peak tamp advancement wth tap, u Tamp p,. OCG (bone base) advancement wth tap, u adv, and overall advancement at full nserton, u nsert adv. (B) OCG at fve successve nserton postons, at nserton depths of 1 5 mm (B2-B6), at whch parameters were estmated.

4 380 SU ET AL. Table 1. Bomechancal Parameters and Varables Parameter Defnton Unt A OCG Artcular cartlage surface area of OCG mm2 a OCR Radus of OCR mm h AC,OCG Thckness of AC of OCG pror to mpact mm Sequental tap number to mpact nsert OCG nto OCR, ¼ 1,2,...,N PE W S, Appled energy densty for tap # mj/mm 2 j Secton regon, nto whch cylndrcal nserton area s dvded for analyss, wth j¼,,..., v m Tap number, up to whch mpact nserton of OCG nto OCR s evaluated, for m ¼ 1, 2,..., N Varable Defnton Unt A adv Increment n host-graft bone nterface area between OCG and OCR durng tap # mm 2 F(t) Contact force appled by the tamp N F p Peak force appled by the tamp durng tap # N I c Impact mpulse of the mpact event of tap # N ms L crack Total crack lengths on artcular cartlage surface of OCG mm N Last tap number for nserton sequence T Impact tme duraton durng tap # ms t Relatve tme for mpact event ms u adv OCG advancement dstance durng tap # mm nsert u adv Overall advancement wth full nserton mm u adv [m] Cumulatve OCG advancement dstance after tap #m mm u Tamp (t) Axal dsplacement of the tamp durng tap # mm AC,OCG u p, Peak axal compressve dsplacement of AC of OCG durng tap # mm Tamp u p, Peak axal dsplacement of the tamp durng tap # mm V AC,OCG Surface chondrocyte vablty of AC of OCG after nserton % adv W Energy advancng OCG nto OCR durng tap # mj Total cumulatve energy delvered to AC of OCG by the completon of nserton mj W AC;OCG nsert OCG W Energy delvered to AC of OCG durng tap # mj Tamp W Energy delvered by the tamp to the sample durng tap # mj W Tamp [m] Cumulatve energy delvered by the tamp to the sample after tap #m mj OCG W S, Energy densty delvered to AC of OCG durng tap # mj/mm 2 OCG W S,m Cumulatve energy densty delvered to AC of OCG after tap #m mj/mm 2 adv W S,j Sectonal energy densty of OCG advancement for secton #j mj/mm 2 e p Peak axal stran of AC of OCG durng tap # mm/mm OCG SW S,N Total cumulatve energy densty delvered to AC of OCG by the completon of nserton mj/mm 2 s p Peak contact stress durng tap # MPa cartlage through tap m, W OCG S;m, was calculated as S m ¼1 WOCG S;. Wth the completon of nserton (when m = N), ths summaton s the total cumulatve energy densty delveredtotheacoftheocg,w AC;OCG nsert. In Study 2, the energy assocated wth each 1 mm of OCG travel nto the OCR was calculated to allow overall assessment of the tapered geometry, snce the OCG samples advanced to a dfferent extent durng the successve taps. To do so, analyss was performed for advancement n fve sequental 1 mm sectons, j ¼,,..., v, wth the frst porton startng at 0 n the z axs. Thus, for each secton, the grafthost nterface area was mm 2 (Fg. 2B). The sectonal energy densty of OCG advancement (W adv S,j ) was calculated as the sum of weghted W adv values for each secton j, parttonng the W adv of each tap accordng to the OCG travel dstance wthn each secton. For example, f durng tap #m, the OCG advanced 1.5 mm, through the fnal 0.2 mm n secton #, all the way (1.0 mm) through secton # and then nto the begnnng 0.3 mm nto secton #v, then (0.2/1.5)W m adv was taken to contrbute to W S, adv, and (0.3/ 1.5)W m adv to W S,v adv, wth W S, adv ¼ (1.0/1.5)W m adv. Statstcal Analyss The effects of DR (0.00, 0.05, 0.10 mm, and 0.10 mm wth modfed OCG geometry) on SW OCG S,, N, V AC,OCG and L crack were assessed usng one-way ANOVA wth Tukey post-hoc test. The effects of DR on F p, s p, I C, T, u Tamp p,, u AC,OCG p,, u adv, e p, W Tamp S,, W OCG S,, W adv S, were assessed usng oneway ANOVA for the frst eght taps (1 8) where data were recorded for more than two groups, and unpared twotaled student t-test for 9 10 where data were from the two groups wth tght ft (DR ¼ 0.10 mm, wth vs. wthout modfed OCG geometry). In addton, the effect of OCG geometry modfcaton, whch was DR ¼ 0.10 mm, wth vs. wthout modfed OCG geometry, on W adv S,j of each secton #j, were assessed usng unpared two-taled student t-test. The relatonshp between L crack and W AC;OCG nsert ; SW OCG S,N for all OCG samples n both studes was assessed by lnear

5 OSTEOCHONDRAL GRAFT INSERTION INTERFERENCE FIT 381 Fgure 3. Effects of nterference ft (DR) and OCG geometry modfcaton on nserton mechancs. (A) Total number of taps (N). (B) Total energy delvered to AC of OCG by the completon of nserton (W AC;OCG nsert ). (C) Vablty of surface chondrocytes (V AC,OCG ). (D) Cumulatve OCG advancement (u adv [m]) as a functon of cumulatve nserton energy (W Tamp [m]), after each tap,, as labeled. regresson. Data are expressed as mean SD. Sgnfcance was set at a ¼ RESULTS Study 1 Tghter nterference ft resulted n more total tap numbers and hgher cumulatve energy delvered to the AC of the OCG by the completon of nserton (Fg. 3A B). Compared to the loose ft group, DR ¼ 0.00 mm, the tght ft group, DR ¼ 0.10 mm, requred two tmes more taps (4.4 vs. 9.7, Fg. 3A) and 25 tmes hgher W AC;OCG nsert (53 mj vs. 927 mj, Fg. 3B) and thus hgher SW OCG S,N. When lookng at u adv [m] as a functon of log(w Tamp [m]), tghter nterference ft presented a rght-shft phenomenon of the sgmod curve (Fg. 3D). Ths ndcated that more energy was delvered to reach the same level of OCG advancement nto the OCR. Durng the ntal nserton taps, tghter nterference ft led to hgher F p, s p (taps #1-3), W OCG S, (taps #2-4), and W adv S, (tap #2), as well as shorter T (tap #1, 3, 4) and lower u adv (tap #1, 2, 4). W Tamp S,, u AC,OCG p,, I C, and e p were generally smlar among loose, moderate, and tght ft groups (Fg. 4A C, Table S-1). Tghter nterference ft resulted n more surface chondrocyte death and cartlage tssue cracks n the OCG (Fgs. 3C and 4D F). The mean values of V AC, OCG were 98%, 94%, 83%, 57% for no load control, loose, moderate, and tght ft groups, respectvely. Study 2 Modfcaton of OCG geometry resulted n fewer total tap numbers and lower W AC;OCG nsert, cumulatve energy delvered to AC of OCG, by the completon of nserton (Fg. 3A and B). The mean value of N decreased from 9.7 to 7.6, and SW OCG S, decreased 72%. When consderng u adv [m] as a functon of W Tamp [m] (on a log scale), OCG geometry modfcaton led to a left-shft of the sgmodal curve (Fg. 3D). At the mddle part of nserton, modfed OCG geometry led to lower W OCG S, (taps #4 6) as well as hgher u Tamp p,,andu adv (tap #6 for both). The other mechancal varables were generally smlar (Fg. 4A C, Table S-1). Surface chondrocyte death and cartlage tssue cracks were less among modfed OCG (Fg. 3C and 4D F). The mean value of V AC,OCG for modfed OCG was 76% compared to 57% for non-modfed OCG.

6 382 SU ET AL. Fgure 4. Typcal effects of nterference ft (DR) and OCG geometry modfcaton on mpact mechancs and cartlage damage. (-v) Study Groups are standard OCG geometry wth OCR that generate () loose, () moderate, and () tght nterference fts, and (v) modfed OCG geometry nto OCR wth tght ft. (A-C) Mechancs of OCG nserton. For 4th tap, tme courses of measured (A) tamp contact force, (B) tamp dsplacement and (C) parametrc plot of tamp contact force and dsplacement, llustratng energy-assocated loop. In (B) are ndcated peak tamp advancement ncrement, dvded amongst OCG advancement, u adv [4], and deduced cartlage compacton, u AC p, OCG [4]. In (C) are ndcated delvered energy values. (D F) En face mcroscopc analyss of effects on cartlage. (D) Reflected lght vew of artcular surface after Inda Ink stanng, showng cartlage damage and cracks. Fluorescence vew of artcular surface after Lve-Dead fluorescence stanng, wth (E) lve cell ndcator n green, and (F) dead cell ndcator n red.

7 OSTEOCHONDRAL GRAFT INSERTION INTERFERENCE FIT 383 Fgure 5. Effect of OCG geometry modfcaton on energy for OCG advancement to defned depths. Data of cumulatve energy densty after ndvdual taps was nterpolated to estmate values at the ndcated postons of fve sectons (-v) of 1 mm n depth, each encompassng the 0 5 mm nserton depth. OCG geometry modfcaton resulted n less sectonal energy densty of OCG advancement n the mddle part of nserton. Modfed OCGs had lower W S, adv, W adv adv S,, W S,v compared to non-modfed OCGs. W adv S,, W adv S,v were not dfferent between the two groups (p ¼ 0.52 and 0.06, respectvely), but the mean value of W adv S,v was lower for modfed OCG (Fg. 5). L crack was strongly correlated wth W AC;OCG nsert and therefore SW OCG S,N (R 2 ¼ 0.93, p < , Fg. 6). The mean value of L crack of tght ft group was mm, 100-fold that (0.11 mm) of the loose ft group, and 6-fold that (2.04 mm) wth OCG geometry modfcaton. Qualtatvely, mct showed dfferences n nterference ft (Fg. 7). The tght ft sample presented the Fgure 6. Correlaton analyss of total crack length on graft cartlage surface (L crack ) wth total energy delvered to AC of OCG by the completon of nserton (W AC;OCG nsert ). Data for all samples, wth loose, moderate, and tght nterference fts of standard samples, and also tght nterference ft of modfed samples are ndcated wth colored symbols. Fgure 7. Qualtatve mct assessment of subchondral bone compacton at graft-host nterface after OCG nserton. (A L) mct mage slces were selected from (A D) Upper, (E-H) Mddle and (I L) Lower porton of the graft-host subchondral bone nterface, n (-) Axal and (-v) Sagttal orentatons. Study Groups are standard OCG geometry wth OCR that generate (A, E, I) Loose, (B, F, J) Moderate, and (C, G, K) Tght nterference fts, and (D, H, L) Modfed OCG geometry nto OCR wth tght ft. Yellow lght box n () and () ndcates the zoom-n area of nterest n () and (v), respectvely.

8 384 SU ET AL. most apparent subchondral bone compacton at the graft-host nterface. In the loose ft sample, some empty space (devod of bone) was vsble at the grafthost nterface. Such space was less for the moderate ft sample, and least for the tght ft sample. The modfed OCG sample exhbted a clearly vsble space wthout subchondral bone contact at the offset mddle part of the graft, and patterns smlar to the non-modfed sample at top and bottom of the graft. DISCUSSION The present study ndcates that small varatons n graft-host nterference ft can markedly affect the bomechancs of OCG nserton and the resultant health of, or extent of damage to, the artcular cartlage. Tghter (OCG - OCR) ft resulted n more taps and more nserton energy for nserton as well as more cell death. Wth the same graft-host ft, modfcaton of the OCG subchondral bone geometry to reduce the nterference n the central part of the graft, leavng the proxmal and dstal regon wth the same nterference ft, reduced the nsertonal energy and subsequent damage to the artcular cartlage. These results thus provde mportant consderatons about the graft-host nterference ft for desgn of surgcal methods and nstruments that acheve mpact nserton of osteochondral grafts, whle preservng the health of the grafted artcular cartlage. The desgn of the present study nvolved a number of tradeoffs. Osteochondral tssue from the adult bovne dstal femur was used as a model system, provdng mature artcular cartlage that s consstently healthy although dfferent from human artcular cartlage. Compared to human artcular cartlage n the dstal femur, bovne artcular cartlage s thnner (h AC,OCG of 1.5 mm n the present study versus mm n human 23,24 ); both human and bovne cartlage exhbt a range of depth-varyng compressve modul and stffness dependng on geometry and loadng condton (e.g., MPa 25,26 and MPa ). The subchondral bone of adult bovnes can be as hgh as 10 GPa, 30 markedly hgher than that of 1 GPa, of humans. 31 In addton, the OCG were small n dameter compared to those used clncally (dameter of OCG autografts typcally up to 10.0 mm and of OCG allografts that could be 15.0 mm or more. 32,33 )n order to generate reasonable numbers of samples for statstcal purposes. In addton, the DR can have a range of values for dfferent nstrumentaton systems. In the present study, the dameter of OCGs was 4.8 mm, and the largest DR was 0.1 mm. The present study utlzed bovne tssue to valdate the expermental methodologes and address the scentfc questons and practcal problems of OCG nserton. Also, the strong correlaton of W AC;OCG nsert and SW OCG S,N wth cartlage tssue damage may be translated to dfferent szes of osteochondral samples. 34 In addton, at completon of nserton, a gap was left between the base of OCG and the bottom of the OCR to facltate bomechancal analyss. Such a gap s not typcal n clncal practce but allowed the expermental desgn of the present study to focus on the process of graft advancement nto the recpent ste. In certan practcal scenaros (f the graft s proud ), mpact load mght be appled, even when the base of the OCG s n contact wth the bottom of the OCR, n order to bottom out the graft. Ths ssue was not addressed n the present study, as such loadng s assocated wth hgh nsertonal loads and cell death. 8 There are a varety of graft-host nterface nteractons that lkely account for the hgher nserton energy requred for tght ft samples. Durng each tap, the appled mpact energy was parttoned to two major components: the AC of OCG, and the OCG-OCR subchondral bone system. Usng a smplfed model of two structures connected n seres, mechancal analyses ndcated that more energy s parttoned nto the component wth the lower stffness. As the OCG advances further nto the OCR, the subchondral bones of both can undergo elastc and plastc deformatons, as well as trabecular fracture at the contact nterface. The magntude of the above ncreases wth ncreasng graft-host nterference ft, thus resultng n a hgher equvalent stffness. Therefore, wth the same W PE S,, hgher DR led to hgher W OCG S, and less u adv. Consequently, the tght ft group requred more taps wth hgher SW OCG S, to complete the nserton, and resulted n more damage to the artcular cartlage. The modfcaton of OCG geometry decreased the equvalent stffness of the graft-bone subchondral bone system by reducng the graft-host nterface contact area. Ths was ndcated by the lower W adv S,j n the modfed OCG group when the OCG subchondral bone advanced nto the OCR (j ¼ to v). Clncally, from the perspectve of bone fracture fxaton, osseous healng can occur wth a small gap even wth slght graft-host relatve moton. 35 The effects of DR and OCG geometry modfcaton on graft stablty after nserton have yet to be elucdated. Graft stablty may be modulated by graft length and dameter, as well as repeated nserton, as quantfed by the pull-out load. 36 However, n vvo after surgery, the OCG and OCR sustan repettve compressve and shear loads, so that a varety of mechancal ndces of stablty are of nterest. Whle the results of the present study ndcate that lower DR facltates OCG nserton wth less mpact energy, such DR may also predspose an OCG to excessve mcro-moton and loosenng. In concluson, the present study provded new nformaton about the effect of graft-host nterference ft on OCG nserton bomechancs and ts bologcal consequences. The optmal graft-host nterference ft thus depends upon the delcate balance between preservng graft cartlage health and adequate postnsertonal graft stablty. Based on the foundatons of these fndngs, further nvestgaton can be carred out toward clncal translaton. The OCG geometry

9 OSTEOCHONDRAL GRAFT INSERTION INTERFERENCE FIT 385 modfcaton may mprove ths balance, mtgatng njury to graft cartlage whle restrctng lateral moton wth a relatvely larger DR and tghter ft at proxmal and dstal portons of the graft. Investgaton of other OCG-OCR heghts and DR would be useful to further characterze the geometry modfcaton. AUTHORS CONTRIBUTION All authors were sgnfcantly nvolved n study desgn, executon of experments, data analyss and manuscrpt preparaton. All authors concur wth the content and approved the fnal submsson of the present manuscrpt. ACKNOWLEDGMENTS Fundng was from NIH RO1 AR (WDB), NIH R01 AR (RLS), NIH PO1 AG07996 (RLS). Natonal Yang- Mng Unversty, Tawan (AWS). Potental Conflcts of Interest are the followng. Board Membershp: Orthopaedc Research Socety (RLS); Consultancy: Jont Restoraton Foundaton (WDB). Stock or Stock Optons. GlaxoSmthKlne (RLS), Johnson and Johnson (RLS), Medtronc (RLS), Moxmed (WDB), OrthAlgn (WDB). Consultancy: Jont Restoraton Foundaton (WDB), a nonproft tssue bank and dstrbutor of osteochondral allografts, DePuy (WDB), Organogeness (WDB), OrthAlgn (WDB). Intellectual Property Royaltes: DePuy (WDB), Smth & Nephew (WDB), Zmmer Bologcs (WDB). REFERENCES 1. Hangody L, Vasarhely G, Hangody LR, et al Autologous osteochondral graftng-technque and long-term results. Injury 39:S32 S Cole BJ, Pascual-Garrdo C, Grumet RC Surgcal management of artcular cartlage defects n the knee. J Bone Jont Surg Am 91: Borazjan BH, Chen AC, Bae WC, et al Effect of mpact on chondrocyte vablty durng the nserton of human osteochondral grafts. J Bone Jont Surg Am 88: Pallante-Kchura AL, Cory E, Bugbee WD, et al Bone cysts after osteochondral allograft repar of cartlage defects n goats suggest abnormal nteracton between subchondral bone and overlyng synoval jont tssues. 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10 386 SU ET AL. and the sze-dependency of cortcal bone modulus. J Bomech 23: Bugbee W, Cavallo M, Gannn S Osteochondral allograft transplantaton n the knee. J Knee Surg 25: Mall NA, Harrs JD, Cole BJ Clncal evaluaton and preoperatve plannng of artcular cartlage lesons of the knee. J Am Acad Orthop Surg 23: Su AW, Chen Y, Dong Y, et al Energy densty and shear are mechancal medators of cartlage damage durng mpact. Trans Orthop Res Soc 39: Claes L, Augat P, Suger G, et al Influence of sze and stablty of the osteotomy gap on the success of fracture healng. J Orthop Res 15: Duchow J, Hess T, Kohn D Prmary stablty of press-ftmplanted osteochondral grafts. Am J Sports Med 28: SUPPORTING INFORMATION Addtonal supportng nformaton may be found n the onlne verson of ths artcle at the publsher s web-ste.