Unilateral face-bows: a theoretical and laboratory analysis.

The investigation evaluated and compared the effectiveness of five face-bow types in delivering unilateral distal forces to their inner-bow terminals. The face-bow types evaluated were bilaterally symmetrical, soldered offset, spring attachment, swivel offset, and power arm. A strain-gauge transducer system was used in the laboratory analysis. A theoretical analysis was also conducted, patterned after the method of Haack and Weinstein, and the results were compared to those obtained from the laboratory analysis. The method used for the theoretical evaluation of the sample face-bows was found to be a reliable means of approximating the percentage distribution of distal forces to the inner-bow terminals. The magnitude and direction of the net lateral forces delivered to the inner-bow terminals was also reliably determined. The results of the theoretical and experimental study of the five face-bow types indicate that the power-arm unilateral face-bow and the swivel-offset unilateral face-bow were effective in delivering a clinically significant unilateral distal force. The bilaterally symmetrical face-bow, the soldered-offset unilateral face-bow, and the spring-attachment unilateral face-bow were not effective in delivering clinically significant unilateral distal forces. Every face-bow effective in delivering unilateral distal forces will also deliver a net lateral force to the inner-bow terminals. This net lateral force will have a direction running from the inner-bow terminal receiving the greater distal force toward the terminal receiving the lesser distal force.