Implantable splint for correcting lumbosacral spondylodesis

A spinal distraction splint has two like anchor screws extending along respective longitudinal screw axes and adapted to be anchored in the pelvis with the axes crossing. Each of the screws has a head formed with a transversely open recess centered on a respective transverse axis and with an annular array of teeth centered on and angularly equispaced about the respective transverse axis. A bolt extends through one of the recesses and is threaded in the other recess. Thus the bolt secures the heads together with the transverse axes coaxial and the teeth interengaged. Respective distraction rods each have one end braced on the respective screw and another end braced oppositely against a vertebra. Once the two anchor screws are properly inserted and then their heads are locked together the interfitting teeth on the heads absolutely locks them against angular rotation relative to each other about the axis of the fastener bolt and all other relative motion is eliminated by the clamping action of this bolt.

FIELD OF THE INVENTION 
The present invention relates to an implantable spinal splint. More 
particularly this invention concerns such a splint which is used to 
correct lumbosacral spondylodesis. 
BACKGROUND OF THE INVENTION 
An implantable spinal splint for treating lumbosacral spondylodesis has a 
pair of anchor rods seated in the ilia and extending at angles to each 
other with their outer ends juxtaposed. An appropriately angled connector 
interconnects these two ends so they form a rigid platform against which 
two distraction rods or braces are propped. The other ends of these braces 
are engaged under the rear processes of a lower-back vetebra to urge them 
up, thereby bending the upper spine portion forward. 
Due to the considerable variation in pelvis size and shape, the angle at 
which the two anchor screws extend relative to each other varies through a 
wide range. Thus it is necessary for the surgeon to carry a stock of 
angled couplings for all the entire range of positions. If screw diameter 
varies, it is also necessary to carry the full range of angular sizes in 
each different diameter. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
implantable spinal splint. 
Another object is the provision of such an implantable spinal splint which 
overcomes the above-given disadvantages. 
Yet another object is to provide a spinal splint which is adjustable but 
which when tightened is extraordinarily rigid and resistant to loosening. 
SUMMARY OF THE INVENTION 
These objects are attained according to the instant invention in a spinal 
distraction splint having two like anchor screws extending along 
respective longitudinal screw axes and adapted to be anchored in the 
pelvis with the axes crossing. Each of the screws has a head formed with a 
transversely open recess centered on a respective transverse axis and with 
an annular array of teeth centered on and angularly equispaced about the 
respective transverse axis. A bolt extends through one of the recesses and 
is threaded in the other recess. Thus the bolt secures the heads together 
with the transverse axes coaxial and the teeth interengaged. Respective 
distraction rods each have one end braced on the respective screw and 
another end braced oppositely against a vertebra. 
Thus with the system of the instant invention, the two anchor screws are 
properly inserted and then their heads are locked together. The use of 
interfitting teeth on the heads absolutely locks them against angular 
rotation relative to each other about the axis of the fastener bolt and 
all other relative motion is eliminated by the clamping action of this 
bolt. The teeth are relatively fine, normally allowing about 10.degree. 
offset between adjacent locking positions, so that the system can fit any 
size or shape pelvis. 
According to this invention the other recess is threaded and the one recess 
is tapered and the bolt has a complementarily tapered head. The threaded 
connection obviously is very strong. Once the bolt is torqued tight, its 
frustoconical head will lie in all-around surface contact with the 
interior of the respective recess, so that an extremely tight connection 
is made, one that is very resistant to loosening. 
The screws of the instant invention are formed around the respective heads 
with seats centered on the respective transverse axes and perfectly 
complementary to the outer surface of the head of the other screw. These 
heads and seats are generally cylindrical and centered on the respective 
transverse axes. Their surfaces also are in tight intimate contact with 
each other once the sacral-rod assembly is tightened so that the two 
screws are snugly secured against any type of motion relative to each 
other. 
The bolt according to the invention has a head formed with an actuation 
recess of polygonal section. Thus it can be tightened with an allen or hex 
wrench.

SPECIFIC DESCRIPTION 
As seen in FIG. 1 an implant for treating lumbosacral spondylodesis has two 
like sacral screws or rods 1 anchored in the ilia 3 at the back of the 
pelvis 2, symmetrical with the skeletal symmetry plane. To this end these 
rods 1 have pointed ends 4 and are formed with screwthreads 5. They are 
centered on respective axes A that cross at an obtuse angle at the 
skeletal symmetry plane and are there connected together at heads 9 and 
9'. 
Braced between each of the rods 1 and a lower-back vertebra 8 is a 
distraction rod 6 having hooks 7 on its ends, one hook 7 engaging under 
the rearward bony process of the vertebra 8 and the other hook 8 engaged 
over the respective screw 1. These rods 6 are of adjustable length between 
their hooks or anchors 7 and may even be of two-part telescoping 
construction with the two parts threaded together so that relative 
rotation steplessly varies length. These rods 6 may also be constructed 
along the lines described in my copending patent application No. 383,169 
filed May 28, 1982. 
As better seen in FIGS. 2-5 the two screws 1 are like but not identical. 
Their heads 9 and 9' are externally complementarily shaped, each having a 
part-cylindrical seat 19 centered on a transverse axis A' perpendicular to 
the respective axis A and a complementary part-cylindrical outer surface 
20 fittable with the seat 19 of the other screw 1 with the axes A' 
coaxial. In addition, the head 9 is formed with a frustoconically tapered 
recess or passage 10 and the other head 9' with a screwthread recess or 
passage 11, both centered on the axes A'. In addition around each of the 
passages 10 and 11 there is an axially centered annular array 12 of 
thirty-six teeth 13 that determine a multiplicity of 10.degree.-offset 
positions for the two screws 1 relative to each other about the coaxial 
axes A'. 
The two complementary heads 9 and 9' are secured together by a locking bolt 
16 having a threaded shank 15 that engages fully in the threaded passage 
11 and a frustoconical head 14 complementary to the passage 10. The screw 
19 also has a hexagonal recess 18 in its head 14 so it can be tightened by 
an allen wrench or hex key. 
When the screw or bolt 16 is driven home it locks the two screws 1 together 
into a single very rigid assembly. The complementary head 14 and recess 10 
will interfit in surface contact once the two heads 9 and 9' have interfit 
with the teeth 13 of one engaging between the teeth 13 of the other, the 
surfaces 19 and 20 juxtaposed, and the axes A' coaxial. The parts 
therefore virtually all fit together in extensive surface contact. If of 
normal metallic construction they can be counted to freeze molecularly or 
cold weld to each other. 
Thus the assembly according to this invention provides an extremely rigid 
platform for the distractor rods 6, so that these rods can exert 
considerable forces. At the same time the use of two joinable rods 1 
allows the orthopedic surgeon to operate without having to have on hand a 
large stock of variously angled couplings.