CA2142390A1 - Interference screw having a double tapered root - Google Patents

Abstract

An dual tapered orthopedic fixation screw is used to anchor ligament grafts inside a bore in a bone mass. The fixation screw includes a root portion having a front section, a cylindrical center section and a back section. A screw thread is formed over substantially the entire root portion to increase purchase. A taper is formed on the back of the root portion to increase compression of the fixation screw against the ligament graft. The screw further includes a rounded portion that is formed between the taper and the back end of the root to prevent the screw from fraying or cutting the ligament.

Description

INTERFERENCE SCR~VV
- HAVING A DOyBLE TAPERED RQ~T

-This inventi~n relates generally to apparatus for securing bone 5 an~l ligament grafts to a bone mass and more particularly to an orthopedic ~ lio" screw for holdin~ the ~ra~E inside a bore.

The a"terior cruciate li~ament (ACL) is frequently injured in conta~t spo~t~ and other activities. Such injuries cause instabili~y in 10 the knee to an extent that ACL reconstruc~ion may be require~.

In ACL reconstruction, 2 substitute li~anlent or graft is cl ,ed to the distal femur or proxirnal tibia to facilitate re~wth and per",~"ent attachment. Various methods of sraft attachment 15 ~re known ~uch as staples and sutures, however, suc~ ~ttachment methods are often not strong enou~h to withstan~ normal stress placed on the ligament.

One ",eti,od for increasing the sttength of the graft attacil ""~
2û comprises v~/edging an inl~, rerei ,ce screw between a ~raft bone block an~ an int~rior wall of a bore formed thou~h the bone rnass.
Although interference screws are stronger than other ligament ~tl~ ent ",e~,Gds such as staples the sharp outer edges of ~he screw often cut or fray the ligament after the scre~ is fixated inside 2~ the bore.

U.S. Pat. No. 5 211 G47 to Schmieding describes a sheath used for protec~in~ a li~ament gr~ft during the insertion process of an interference screw. The she~th contains a cutout portion that 30 enables the interference screw to be exposed to ~he tunnel wall of the bone during insertion while at the sanle tirne covering the side of the screw facing the ~raft. The intei rere"ce screw contains a rounded back section to fur~her r~duce the effects of graft cut~ing.

Because there ars no threads ~t the back end, the screw shown in Schmieding has reduced purchase over fully threaded screws of comparable size. Reduced purch~se in the s~rew increases the chance that the ligament ~raK may slip in the bore ~4;~3~30 Further, the screw in Schmieding h~s a cylindrical root portion that provides only nominal compression of the ~raft against the inside all of the bore.
, A constant root diameter also exhibitS substantially constant resistance when inserted by a surgeon into the bore. Thus, it is hard for the sur~eon to physically ~ause exactly how ~r the interferen~e scre~lv has been inserted into the bore. For ex~mple, it is important that the scre~ is not inserted too far past the back end of the bore, while at the same time i",p~lan~ tha~ the screw does not extend partially out of the bore after cornpletion of the installation process. The scre~,v in Schmiedin~. however, ~ives no physical indi~tion that insertion is near completion.

Accordingly, a need remains for a hi~h purchasc interference screw that will not cut or fray an attached 1i3~ment graft.

It is, thcrefore, an object of the invention to increase the pu~ctlase of interference screws.
~nother object of ~he invention is to reduce fraying or cutting effects caused by interference scre~vs on bone nnd li~ament grafts.

A further object of the invention is increase the stren~th in which grafts c~n be attached to a bone mass.

Accoldi"y to the invention we provide an orthopaedic fixation ~cre~v for a- Icl ,oring a bone ~raft in a bore forme~ in a bone mass, said screw cornprising a root portion havin~ a front ~nd back end, the root portion includin~ a center section and ~ ba~k se~tion;
~ screw thread formed over ~ubstantially all of the center and back sections; and a taper formed on said root back section and extending - -substantially entirely between said back end and said center section, said taper being la~ger tow~rd the back end than toward the cente~ section.

We further provide an orthopae~ic intel F~r~nce screw for - compression ~llc~l~ri~ ,3 a bone ~r~ft in a bore fo~n~ecl in a bone mass, said screw comprisin~:
a root hav;n3 front and back ends and further including at least 5 a center se~tion and a back section;
a ~crew thread formed over substantially all of the center and ~
~aGhir~y ~;~CtiGil5, the screwthread inclu~in3 a c~estwith an outer surface havin3 a wide width that increasss at ieast along the back section away from the c~nter gection; and a taper formed on said root back section and extending slubstantially entirely between said back end and said center section, said taper being larger tow~rd the back end than toward the center section.

An o~thopedic fixation screw contains a specially shaped root portion that anchors bone srafts more securely inside bores in a bone mass. The screw is less likely to cut a li~r"~nt ~raft and also provides a positive posttional indication to a surgeon when inserting the suew inside the bofe.
The screw ino~udes a root portion havin3 a front and bac~c end.
The root poftion inch~des a front section a cylindrical center section and a back section. A screw thread is formed over substantialiy the entire root portion for inueased pu,.;i,ase capacity.
Pre~rably the root back section includes an outer surface the ~aGk encl outer surf~ce having a rounded edse be~veen said back end outer surface and the outer suRace of the root back sectlon.

r, eferably the root diameter is substantially constant ~lon~ the center se~tion, thereby ~aintainin~ a substantially uniform ~iarneter across the th~eacl along the center ~ection of the root.

Preferably the distance between the screw thread crest and the outer surFace of the root alon~ substantially the entire back section diminishes toward the back end. - ~ ~~

The disl~l ,ce between the thread crests ~n~ the outer surface - of ~he root alon~ the b3~k root section rnay ~iminish inversely tosaid taper, thereby nlaintaining a s~,bslanlially uniform di~meter across the threacl aiong the b~ck section of the root.

The root may further include a front section h3ving a taper, said front section taper extending substantially entirely alon~ the front section, such that the front section root diamater deueases away from the center section.
The screw thl e~d crest may extend above the outer surface of the root, the distance between ths screw thread crest and the outer su~Fa~e of the r~ot along the ~enter section is substantially C6~ dl It ~iong the ~enter section, thereby maintaining a substantially uniform 15 diameter acros~ the thread along the center section of the root.

A taper is f~""ed on the back ssction to increase compr~ssion a~ainst the ~raft. The screw further includes a rounded portion formed between the taper and t~le back end o~ the root that prevents 20 the ligament from being cut or frayed after the screw is fully inserted.

The thread maintains ~ un jform crest diameter along the back and middle sections. The crest of the thread also has a top fac~
25 that increases in width toward the back end of the root portion. The root portion can either be cannul~ted for receiving insertion gui~le wires or can have a solicl center with a rounded front end.

The foregoing and other obj~cts, fQatures and advantages of 30 the invention will become mor~ readily ~pparent from the foliowin~
detailed description of a preferred embo~irnent of the invention which proceeds with reference to the a~companying drawin~s.

FIG. 1 is a side section view o~ a fi~alioi, screw accor~li"~ to 35 the inventi~n.

FIG. 2 is a rear view of the fixation screw sho~n in FIG. 1.

~14~2390 Fl~. 3 is a side section view of the fixation screw shown in FIG. 1 p~rtially inserted into a bore bet Neen an interior wall of a bone mass and a ligament graft.

FIG. 4 is the side sectlon view of FIG. 3 with the fixation screw fully inserted into the bore.

FiG. 5 is an all~",ative embodiment of the fixation scre~
according to the invention having a solid root portion.
Flt;. 6 is a rear view of the fixation screw shown in FIG. 5.

FIG. 1 is a side se~tion vie~v o~ an o~hopedic fixation ~crew~12~
a~cording to ~he invention. The rixalion screw 12 has a ~ y~
root portion 14 with a circular cross sectional shape. The root portion 14 includes a front end 26 and a baGk end 28 and is ~eneraily defined by a front section 16, a center section 18 and a back ssction 20. A thread 22 is formed over s!,~bst~ntially the entire root port;on 14 from the front end 26 to the back end 28.
A taper 24 is fun~,~d on the back section 20 between the back end 28 and the center section 18. The tapcr 24 includes an outside su~face 25 that extends laterally olJt from the root po~tion 14 at an an~le 30. The an~l~ 30 of outside face 25 ~nakes the taper larger toward the b~ck en~ 28 th~n toward the center section 18. In one embodiment, the outside surface 25 of taper 24 has an slop~ of approximately 7 degrees in relation to an ~utside surface 36 of root 14. A rounded portion 32 i8 formed between the taper 24 and the back end 28 of root 14.
The center section 18 has a cylindrically shape. The front section 16 is tapered from the front end ~6 to the center section 18 and has a larger diameter towar~ th~ center section 18 than to~ard the front end ~. The fr~nt, center an~ back sections of the r~ot 36 portion 14 each have substantially the same length.

The thread 22 includes a crest 3~ that extends a ~iven hei~lt above the outside surf~ce 3~ of root 14. The crest hei~ht 23~0 diminishes between the center section 18 and the back end 28 - invers~ly to the slop~ 30 of taper 24. Thus, a substantially uniform crest diameter 40 is maintained along the back section 20 and - - -~ middle section 18. The crest diameter 40 narrows slightly along the 5 front of the scre~v in proportion to ~he ~aper on front section 16. The cJe~t 38 in~ludes a top face 3~ tl~at in~reases in ~idth to~lvard the back end of the root as is sho~n in FIG. 3.

In one e"l~odi",ent~ the root portion 14 i5 eannulated for receivin~ ~uides wires that direct the screw to a desired mountin~
p~sition. P~eferrin~ both to FIG. 1 and to the back view of the fixation ~creu~ ~2 sho~n in F~G. 2, the cann~lated ~oot po~tion 14 includes a hexasonal socket 42 for en~a~ing a screw driver (not shown). A center channel section 46 and a front channel seGtion 48 15 each have circular cross sectional shapes that combine to Gxtand thro~gh the center section 18 and front sect'on 16, respectively.

FIG. 3 is a side section view of the fixation screw 12 in FIG, 1 shown pa~tially inserted into a bore 54. In the case of ACi 2~ recons~ructive sur~ery, the bore is formed in the distal fem~r or proximal tibia, or both. Screw 12 is used to anchor a bone-tendon graft 56 inside bore 54 which is also referred to as an endosteal tunnel. Graft 56 includes a tendon 60 and a bone portion 62.

2$ An o~thopedic ~u~eon drills into bone mass 52 usin~ a convention rJ, l~ ,o,tJediG drili and which may be done endoscopiGally.
After bore 54 i5 drilled as shown in FIG.3, one end of ~r~ft 56 is positioned in the bore as illustrated. Screw 12 is fitted onto a driver (not show) by inserting ~ hexa~onal f~ont end of the driver into 30 socket 42 (FiG. 1). The surgeon grasp~ the driver and positions the front end 26 of screw 12 between an interior wall 58 of bone mass 52 ~nd tendon 60, The surgeon pushes the driver axially toward screw 12 while at 35 the same rotating the driver. The ll ,re~ds 22 in turn en~age both the inner wall 58 ancl ligament 60. As the frcnt end of screw 12 advances further into the space between ligament 60 and wali 58~
the tapered front section 16 (FIG. 1 ) gradually compresses ~raft 56 ~14~3~0 and bone po~tion 82 a~ainst ~he opp~site side of bore 54.
Compression againsS ~raft 56 is steadily increased as screw 12 is inserted further into bore 54. The sc~ew ~ is inserted until tt~e back section 20 (FIG. 1 ) teaches the front of bore 54 as shown in FIG. 3.

l~efen~i, IsJ to ~IG. 4, as the tapered back section 20 begin~ -insertion into bore 54, the rate in which the screw 12 c~",~resses a~inst ~r~ft 56 i"c, ~ases. More force is thereby roquired by the surgeor~ to further insert the remainin~ portion of sc~ew 12 into ~ore 10 54. The added l~:sisLance from screw 12 glves the s~rgeon positive feedback as to how far the screw i8 presently inserted into bore 54.
The tape~ ~4 on the back por~ion of root 14 al50 increases pu~chase over stal~dar~l cyli~dri~l roots. Thus, 8uew 12 holds ~raft 56 more seclurely inside bore 54 than other screws of comparable size. 5 The top face 39 increases in width toward the back of screw 12. Each tum of crest 38 thereby cuts a wider groove into wall 58 th~n the proceeding turn. The threads in the ~ack of 8crew 12 will then not sit loosely in ~rooves pre~iously bol e~ by proceedin3 20 threads. Thus, each threa~l is held flrmly in bone mass 52 further red~cin3 the possibility of screw 12 coming looso.

The tapcrgd front root section 16 allow the thre2ds 22 to ~ain purchase at a ~elatively n31 row portion of the root in turn ~educing ~5 shifting or twisting of gl aft 56 during installation. Therefore, it is not neces$ary to p~ovide a cannulated root portion 22 and ~uide wires that would normally be used to prcv~nt screur ~i~ergence during installation.

Screw 12 is further inserted until the back en~ 28 is fully seated inside bore 54 as illustrated in FIG. 4. The rounded portion 32 of screw 12 prevents cuKing or frayin~ of li~an~ent 60 when ent 50 is flexed during regular le~ movement$. Once the screw 12 is positioned as shown in FIG. 4, the sur~eon withdl~aws - -the driver from socket 42.

FIG. 5 shows a screw 64 havin~ a solid root portion 70. FIG. 6 is a back view of the fixation screw 64 showin~ a hexa~onal socket ;~4~3~tO

68 for re~eiving a d~iver ~not shown). Serew ~4 ineludes the same - tapel 8 in the front and back ~ections of the root portion 70. A
rounded portion 66 i8 located at a front end B5.
, Having .lese, iL ed and illustrated the principles of the invention in a prefe"ed embodi~"ent thereof, it ~hould be apparent that the invention ean be modified in arran~ement and detail without d~pal liny from sueh prineiples. I elaim all modifieations and vari~tion eoming within the spirit and seope of th~ following elaims.

Claims (14)

1. An orthopedic fixation screw for anchoring a bone graft in a bore formed in a bone mass, said screw comprising:
a root portion having a front and back end, the root portion including a center section and a back section;
a screw thread formed over substantially all of the center and back sections; and a taper formed on said root back section and extending substantially entirely between said back end and said center section, said taper being larger toward the back end than toward the center section.

2. An orthopaedic interference screw for compression anchoring a bone graft in a bore formed in a bone mass, said screw comprising:
a root having front and back ends and further including a center section and a back section;
a screw thread formed over substantially all of the center and back sections, the screw thread including a crest with an outer surface having a width that increases at least along the back section away from the center section; and a taper formed on said root back section and extending substantially entirely between said back end and said center section, said taper being larger toward the back end than toward the center section.

3. A screw according to claim 1 wherein said screw further includes a rounded portion formed between said taper and said back end.

4. A screw according to claim 1 wherein the distance between the thread crests and the root along the center section is substantially constant.

5. A screw according to claim 4 wherein the distance between the thread crests and the root portion in the back section diminishes between the center section of the root portion and the back end.

6. A screw according to claim 5 wherein the crest height over the back root section diminishes inversely to said taper thereby maintaining a substantially uniform crest diameter along the back and middle sections of the root portion.

7. A screw according to claim 3 wherein said root portion further includes a front section having a taper formed thereon, said front section taper extending substantially entirely between said front end and said center section and being larger toward the center section than toward the front end.

8. A screw according to claim 1 wherein the crest of the screw thread includes a top face having a width that increases toward the back end of the root.

9. A screw according to claim 1 wherein the root portion is cannulated.

10. An orthopaedic interference screw for compression anchoring a bone graft in a bore formed in a bone mass, said screw comprising:
a root having a front section, a center section and a back section;
a screw thread formed over substantially all of the front, center and back sections, the screw thread including a crest with an outer surface having a width that increases at least along the back section away from the center section; and a taper farmed on said root back section and extending substantially entirely between said back end and said center section, said taper being larger toward the back end than toward the center section.

11. A screw according to claim 10 wherein the front and back sections are substantially the same length.

12. A screw according to claim 10 wherein the taper has a slope of approximately 7 degrees.

13. A screw according to claim 10 including a channel extending longitudinally throush the root from the front end to the back end, the channel having a hexagonal socket in the back section for ensaging a driver and a circulat cross-sectional shape-in the center and front sections.

14. A screw according to claim 10 including a rounded nose section joined to the front end of the root.