After placement of multiple dental implants, a radiograph is usually taken to demonstrate the implants' relative positions. The implants can be relatively parallel or not parallel. There may be a certain esthetic satisfaction in parallel placement. There may be prosthetic fabrication issues if the implants are extremely off from parallel. Nonetheless, from a biomedical engineering perspective, nonparallel placement may better resist occlusal loads.1 

True parallel placement may be impossible when implants are placed in the curved edentulous arch, especially when there is a facial concavity. The importance of parallel placement may be overrated since there are techniques to correct difficulties in prosthetic fabrication.2,3 

Nonparallel placement of multiple implants may increase the stability of a prosthesis when the implants are splinted in the prosthesis. Under load, this placement distributes stresses through more of the containing bone than parallel implants. In-line implants may induce screw loosening under functional load, especially when there is a cantilever,1 

The engineering school of thought asserts that tripodal placement may be best to resist occlusal loading.1,4  When there is a load, any stress on a nonparallel implant will be offset by another nonparallel implant that is directed more axially in the vector path of the load.

Nonetheless, parallelism may be important in removable overdentures for ease in fabrication and to prevent wear issues of the retainers.5 

Parallelism may not be depictable on some panoramic radiographs. Plane film radiographs may not accurately depict the actual position of in situ implants.68 

Parallelism may be desirable for ease of prosthetic fabrication, but parallelism is not mandatory for a successful treatment outcome. Thus, if a dental implant surgeon is criticized for nonparallel implant placement, the response should be that of an adherent of the engineering school of thought.

References

1
Renouard
F,
Rangert
B.
Biomechanical risk factors
.
In
:
Risk Factors in Implant Dentistry
.
Chicago, Ill
:
Quintessence Books
;
1999
:
44
45
.
2
Nunes
DB,
da Silva
P,
Pereira-Cenci
T,
Garbin
CA,
Schuh
C,
Boscato
N.
Correction of nonparallel implants for an implant-retained overdenture
.
Gen Dent
.
2010
;
58
:
e168
e171
.
3
Dario
LJ.
A maxillary implant overdenture that utilizes angle-correcting abutments
.
J Prosthodont
.
2002
;
11
:
41
45
.
4
Weinberg
LA.
Reduction of implant loading with therapeutic biomechanics
.
Implant Dent
.
1998
;
7
:
277
285
.
5
Gulizio
MP,
Agar
JR,
Kelly
JR,
Taylor
TD.
Effect of implant angulation upon retention of overdenture attachments
.
J Prosthodont
.
2005
;
14
:
3
11
.
6
Pramono
C.
Surgical technique for achieving implant parallelism and measurement of the discrepancy in panoramic radiograph
.
J Oral Maxillofac Surg
.
2006
;
64
:
799
803
.
7
Schulze
RK,
d'Hoedt
B.
Mathematical analysis of projection errors in “paralleling technique” with respect to implant geometry
.
Clin Oral Implants Res
.
2001
;
12
:
364
371
.
8
ten Bruggenkate
CM,
van der Linden
LW,
Oosterbeek
HS.
Parallelism of implants visualised on the orthopantomogram
.
Int J Oral Maxillofac Surg
.
1989
;
18
:
213
215
.