A guide system for resecting a bone surface in a desired location comprising: a surface trial having at least one hole (11, 12); a centering guide post inserted into the at least one hole (11, 12) of the surface trial and releasably engaged to the surface trial, the centering guide post having a first location on its length with a maximum and a minimum cross sectional dimension, the cross sectional dimension decreasing when subject to external forces applied to the centering guide post; and a centering guide handle attached to the centering guide post, wherein the centering guide handle positions the surface trial on the bone surface, wherein the centering guide post is cannulated (135, 435) so that it is open at both ends of its length, the centering guide post further comprising a slit separating the centering guide post into first and second flexible leg segments, each of the first and second flexible leg segments having an attachment portion, the attachment portion having a decreased size relative to other parts of the post, and the first and second flexible leg segments being flexible such that when external forces are applied to outer surfaces of the centering guide post (130 along the length of the centering guide post corresponding to its longitudinal axis, a distance between the leg segments decreases and the leg segments move closer together, thereby decreasing the cross sectional dimension of the centering guide post, wherein the flexibility of the centering guide post at the first location allows insertion of the centering guide post into the at least one hole (11, 12) of the surface trial, the attachment portions of the first and second flexible leg segments having a geometry and size to be positionable within the at least one hole (11, 12) of the surface trial and the centering guide post becoming releasably engaged to the surface trial when inserted into the at least one hole (11, 12), wherein at the first location, a space including the slit between the leg segments traverses the cross section of the centering guide post, said space having a varying dimension along a length of the space, wherein the slit extends from a free end of the centering guide post to a second location, the distance from the free end to the second location being less than the length of the centering guide post, and wherein the leg segments begin at the second location near the connection between the centering guide post and a shaft and extend to the free end of the post, each leg segment increasing in size over a first distance from the first location to a location between the first location and the free end, but abruptly decreasing in size over a second distance from the location in between the first location and the free end toward the free end, the shaft being in between the centering guide post and the centering guide handle. a surface trial having at least one hole (11, 12); a centering guide post inserted into the at least one hole (11, 12) of the surface trial and releasably engaged to the surface trial, the centering guide post having a first location on its length with a maximum and a minimum cross sectional dimension, the cross sectional dimension decreasing when subject to external forces applied to the centering guide post; and a centering guide handle attached to the centering guide post, wherein the centering guide handle positions the surface trial on the bone surface, wherein the centering guide post is cannulated (135, 435) so that it is open at both ends of its length, the centering guide post further comprising a slit separating the centering guide post into first and second flexible leg segments, each of the first and second flexible leg segments having an attachment portion, the attachment portion having a decreased size relative to other parts of the post, and the first and second flexible leg segments being flexible such that when external forces are applied to outer surfaces of the centering guide post (130 along the length of the centering guide post corresponding to its longitudinal axis, a distance between the leg segments decreases and the leg segments move closer together, thereby decreasing the cross sectional dimension of the centering guide post, wherein the flexibility of the centering guide post at the first location allows insertion of the centering guide post into the at least one hole (11, 12) of the surface trial, the attachment portions of the first and second flexible leg segments having a geometry and size to be positionable within the at least one hole (11, 12) of the surface trial and the centering guide post becoming releasably engaged to the surface trial when inserted into the at least one hole (11, 12), wherein at the first location, a space including the slit between the leg segments traverses the cross section of the centering guide post, said space having a varying dimension along a length of the space, wherein the slit extends from a free end of the centering guide post to a second location, the distance from the free end to the second location being less than the length of the centering guide post, and wherein the leg segments begin at the second location near the connection between the centering guide post and a shaft and extend to the free end of the post, each leg segment increasing in size over a first distance from the first location to a location between the first location and the free end, but abruptly decreasing in size over a second distance from the location in between the first location and the free end toward the free end, the shaft being in between the centering guide post and the centering guide handle.