ORIGINAL ARTICLE
Principle and stability of locking plates
Ralf Gutwald, Brian Alpert1 and Rainer Schmelzeisen
Department of Oral and Maxillofacial Surgery, University of Freiburg, Freiburg, Germany, and1Department of Surgical and Hospital Dentistry, University of Louisville, Louisville, KY, USA(Received for publication on September 1, 2002)

Abstract. A new internal Mini-Locking-System was tested compared with conventional 2.0 mm Miniplates. Standardised osteotomies in the angular region of 16 human cadaver mandibles were ﬁxed
with a 6-hole-plate at the oblique line. Osteosynthesis and stability of ﬁxation was proofed in a threedimensional in-vitro-model in which functional load was simulated. Comparison of the different
osteosynthesis techniques showed that in the case of Miniplate ﬁxation torsion and gapping of the bone
fragments occurred following plate application and screw tightening when the plates were pressed onto
the bone, so last incongruences between bone surface and plate were transferred to the mobile bone
fragments resulting in more extended gaps and torsion. This was only observed to a much lesser extent
with the Mini-Locking-System due to the ﬁxation principle avoiding pressure to the bone. Duringfunctional loading the Mini-Locking-System showed also a signiﬁcant higher stability in comparison toconventional Miniplates. Due to the ﬁxation method imitating the principles of a ﬁxateur the screws
form together with the plate and the cortical bone a frame construction. Loading forces are transmitted
without the need of plate friction directly from bone over the screws to the plate resulting in higherstability. (Keio J Med 52 (1): 21–24, March 2003)
Key words: miniplate, osteosynthesis, locking-system, mandibular angle fracture

Introduction

Materials and Methods

With the development of osteosynthesis in maxillofacial surgery different systems have been designed
which have become smaller, more simple to handle andavoid extraoral procedures. Meanwhile Miniplate ﬁxation of mandibular fractures has become a standard
treatment.1 Nevertheless complications in Miniplate
osteosynthesis of the mandible especially in the angularregion have been described in up to thirty per centof cases, like movement and loosening of screws following failure of fracture treatment.2 Also plate pressure following disturbance of the blood supply andbone necrosis especially around the screws will lead toloss of plate friction.
In order to improve Miniplate osteosynthesis anew internal Mini-Locking-System has been developedin collaboration with the AO/ASIF-Institute (Davos,Switzerland).

Dimensions of the new Mini-Locking-System arecomparable with conventional 2.0 mm Miniplate systems. In addition the selftapping 2.0 mm locking screw
has a special double-lead thread beneath the screw
head. The locking plate has corresponding threaded
plate holes. During insertion the locking screw engages
and locks into the threaded plate holes (Fig. 1).
In a preclinical investigation at the AO/ASIFInstitute (Davos, Switzerland) we proofed the handlingand mechanical stability of the new Mini-LockingSystem compared with the conventional 2.0 mm Miniplate system.
Standardised osteotomies in the right angle region of
sixteen human cadaver mandibles were performed andﬁxed with one six-hole-plate (length 40 mm, thickness
1.0 mm) and 6 screws of each system (length 6.0 mm,outer diameter 2.0 mm) at the oblique line. Eight man-Presented at the 1267th Meeting of the Keio Medical Society in Tokyo, July 15, 2002.
Reprint requests to: Dr. Dr. Ralf Gutwald, Department of Oral and Maxillofacial Surgery, University of Freiburg, Hugstetterstrasse 55, 79106
Freiburg, Germany, e-mail: gutwald@zmk2.ukl.uni-freiburg.de21

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Gutwald R, et al: Principle and stability of locking plates

Fig. 3 Gaps and torsion following osteosynthesis. White bars ¼
Mini-Locking-System. Black bars ¼ Miniplate.
Fig. 1 Mini-Locking-System (plate thickness 1.0 mm, screw outer
diameter 2.0 mm).

Fig. 2Three-dimensional in-vitro-model.

dibles were ﬁxed with the Mini-Locking-System andeight mandibles with conventional Miniplates. To exclude the inﬂuence of anatomic individuality the mandibles were prior divided concerning the match-paired
method resulting in two equivalent test groups.
Following osteosynthesis gaps and torsion at theosteotomy site (cranial and caudal) were measured.
The mechanical stability of the different ﬁxation
methods was tested according to Kroon’s method3
based on a three-dimensional in-vitro-model (Fig. 2).
Application of functional load by 20, 35, 50 and 65 N
was performed at nine different positions along the rowof teeth. The functional load was applied consecutively
without tightening the screws for each load. Changes ofdimension as a reaction to distraction or compression
were computer-assisted recorded at three locations of
the osteotomy (cranial, lingual, caudal) by means ofstrain gauges.

Fig. 4 Osteotomy mandibular angle ﬁxed with Mini-Locking-System
during functional loading. Typical dislocation at the caudal borderof the mandible if the load was applied near to the osteotomy(arrow ¼ loading point 6).

Statistical evaluations were performed using analyses
of variance (General mixed model analysis of variance,Mann-Whitney-Test).
Results
Following osteosynthesis the mandibles treated withthe Mini-Locking-System showed the lowest gaps andtorsion. Compared with the Miniplates the differences were signiﬁcant (Fig. 3). At the caudal site of theosteotomy the highest level of signiﬁcance was found(p ¼ 0.010).
During mechanical loading simulating the muscle
forces we observed in both groups the typical increasing
dislocations at the caudal border of the mandible if theload was applied nearer to the osteotomy (Fig. 4). Ex-Keio J Med 2003; 52 (1): 21–24

23Fig. 6 Primary stability of conventional Miniplate. Arrows ¼ load
distribution and pressure/counter-pressure (friction) at the plate/bone.

Fig. 5 Distraction and compression at the caudal site of the osteotomy (loading with 35 N). White bars ¼ Mini-Locking-System. Blackbars ¼ Miniplate.

cept increasing dislocations no differences concerning
the characteristic of the value distribution was found byincreasing the functional load.
Looking on the different ﬁxation methods the MiniLocking-System showed the best stability. In relation tothe four different loading forces the values in all loading-points were signiﬁcant lower for the Mini-LockingSystem compared with the Miniplates. The highest levelof signiﬁcance (p ¼ 0.015) was detected at the caudal
strain gauge (Fig. 5).

Fig. 7 Primary stability of Mini-Locking-System. Arrows ¼ load distribution (transmission directly from the bone to the screws, fromthere to the plate and on the opposite side of the fracture again
through the screws into the bone). Rectangles ¼ frame constructs as
result of screws locked into the plate as well as into the bone.

Discussion
Comparison of the different osteosynthesis methods
showed that in case of Miniplate ﬁxation increased torsion and gapping of the bone fragments occurred during
screw tightening when the plates were pressed onto thebone.
This is the result of the different ﬁxation method.
When using conventional Miniplates it is essential tocontour the plate precisely to the bone surface. Otherwise last incongruences between bone surface and plate
will be transferred to the mobile bone fragments during
tightening of screws resulting in more extended gaps
and torsion and will lead to primary loss of reduction.
If the Mini-Locking-System is ﬁxed with locking
screws reduction remains nearly unchanged. Therefore
the plate does not have to be contoured as precisely aswith conventional plating systems.
During mechanical loading we observed in bothgroups the typical increasing dislocations at the caudal
border of the mandible if the load was applied nearer tothe osteotomy.3,4On the other hand, the Mini-Locking-System showed
the best stability. This is also the result of the different
ﬁxation methods.

With conventional technique tightening of screws
presses the plate against the bone. This pressure generates friction which contributes signiﬁcantly to primarystability (Fig. 6). At the screw thread only the side orientated to the plate is involved developing the pressureof the plate.
By the Mini-Locking-System loading forces aretransmitted directly from the bone to the screws, fromthere to the plate and on the opposite side of the fracture again through the screws into the bone. Friction
between plate and bone is not necessary for stability.
On each fracture side the screws locked into the plate
as well as into the bone result in a frame construct withhigh stability (Fig. 7). Forces to the screw are morefavourably distributed over the whole thread contour.
In comminuted or defect fractures ﬁxation with conventional plating systems can lead to secondary dislocation as soon as the pressure between plate and bone
is no longer guaranteed. Plate ﬁxation with locking
screws can avoid this kind of secondary dislocation.
Also in poor quality bone anchoring of screws can leadto screw loosening and subsequent loss of reduction.
Due to the secure locking of the screw in the plate thisproblem can be avoided with the Mini-Locking-System.

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Gutwald R, et al: Principle and stability of locking plates

Fig. 9 Postoperative x-ray of a double-sided fracture of an atrophic
mandible. Fixation with two Mini-Locking-Plates (medium). In addition a left zygoma fracture ﬁxed with three Mini-Locking-Plates(small).
Fig. 8 Pre- and postoperative x-rays of a severe dislocated fracture,mandibular body left. Fixation with two Mini-Locking-Plates (smalland medium).

Conclusion
The new internal Mini-Locking-System combines
two principles including several advantages. The locking principle prevents stripping as well as movement
and loosening of screws. The ﬁxation technique imitates
the principles of a ﬁxateur. This simpliﬁes plate bending
and decreases torsion or opening at the fracture site.
The increased stability is a result of the frame construct
and load transmission as described above. The absenceof pressure underneath the plate prevents interferencewith the vascular supply of the bone and allows periosteum growing under the plates supporting fracture
healing. This was observed clinically in all with MiniLocking-Plates treated fractures during plate removal
after a period of six months.
In addition to the small Mini-Locking-Plates, 2 larger
plate proﬁles were designed accommodating a wide
range of indications. The 2.0 mm screw ﬁts all plateconﬁgurations and can be used mono- or bicortical. If

necessary the threaded plate holes also accept nonlocking screws which permits greater angulation.
First clinical experience with all 3 plate proﬁlesshowed excellent results. Especially in severe and comminuted fractures (Fig. 8) as well as in fractures of
the atrophic mandible (Fig. 9) osteosynthesis with theMini-Locking-System meanwhile has become a standard treatment in our clinic.
References
1. Schmelzeisen R, McIff T, Rahn B: Further development of titanium miniplate ﬁxation for mandibular fractures. Experience
gained and questions raised from a prospective clinical pilot study
with 2.0 mm ﬁxation plates. J Craniomaxillofac Surg 1992; 20:
251–256
2. Ellis E 3rd, Walker L: Treatment of mandibular angle fracturesusing two noncompression miniplates. J Oral Maxillofac Surg
1994; 52: 1032–1036; discussion 1036–1037
3. Kroon FH, Mathisson M, Cordey JR, Rahn BA: The use of miniplates in mandibular fractures. An in vitro study. J Craniomaxillofac Surg 1991; 19: 199–204
4. Schierle HP, Schmelzeisen R, Rahn B, Pytlik C: One- or two-plateﬁxation of mandibular angle fractures? J Craniomaxillofac Surg
1997; 25: 162–168