delivered the opinion of the court:
Plaintiff, Philip Flanagan, sued defendant, Dr. Luis G. Redondo, for medical malpractice in the treatment of plaintiff’s broken leg. Plaintiff appeals from the judgment entered on the jury’s verdict in favor of defendant, arguing that the verdict was contrary to the evidence and trial errors require a new trial.
On July 24, 1982, plaintiff injured his leg when he fell from a ladder. Plaintiff’s wife took him to the emergency room at Christ Hospital, where Dr. Edward Snyder, the orthopedic surgery resident, diagnosed the injury as a spiral fracture of the lower third of the tibia. Dr. Snyder found that plaintiff’s foot was somewhat rotated out of position. He attempted to correct the rotation before he applied a cast. He was dissatisfied with the initial casting, so he removed the cast. After further attempting to correct the rotation, he applied a second cast and admitted plaintiff to the hospital.
Dr. Snyder told defendant, the attending orthopedic surgeon, that he had been unable to totally correct the deformation of the leg. Defendant saw plaintiff the next morning and reviewed the X rays of plaintiff’s foot. He found no indication of abnormal rotation, so he left the cast in place.
During several outpatient visits, plaintiff complained to the defendant that his foot was rotated out from his leg. Both plaintiff’s wife and a friend who testified at trial had also noticed that plaintiff’s foot was obviously cocked out to the side. Eventually, defendant told plaintiff that his leg had healed properly and that he was walking improperly.
*959Dr. Irwin Feinberg, another orthopedic surgeon, examined plaintiff on February 7, 1983. He found that plaintiffs foot was abnormally rotated out. Dr. Feinberg recommended that plaintiff submit to a surgical procedure to correct the rotation. Plaintiff agreed and Dr. Feinberg performed the surgery. During surgery, Dr. Feinberg cut through the upper third of both the tibia and the fibula, realigned the bones and put a new cast on plaintiff’s leg.
At trial, Dr. Feinberg testified, as an occurrence witness for plaintiff, that he cut through the tibia and fibula because that allowed easier manipulation of the bone fragments. He said that an intact fibula does not prevent rotation due to a spiral fracture like plaintiff’s. Defense counsel showed Dr. Feinberg X rays of plaintiff’s leg taken in Dr. Feinberg’s office. Dr. Feinberg testified that although he had not noticed it before, he now saw a fracture at the head of the fibula on one X ray, in addition to the fracture of the lower part of the tibia. He presumed that the fibula head was fractured at the same time as the tibia, unless some other cause could be documented.
Defendant moved to prevent plaintiff’s expert, Dr. Jeffrey Gilíes, from testifying that the X rays showed a fracture of the fibula and from giving his opinion regarding the significance of the fracture and the rotation of plaintiff’s foot. Although Dr. Gilíes’ deposition was not presented to the trial court, plaintiff admitted that Dr. Gilíes had not included a broken fibula as a basis for his opinion that the foot was rotated out of position.1 The trial court sustained defendant’s objection to the prospective expert testimony, stating:
“You take the facts as they are. It might be fortunate or unfortunate as it were that Dr. Feinberg looked at that film and detected there was a fracture at another place he hadn’t seen before. We can’t try that issue here.”
Dr. Gilíes testified that the X rays showing a spiral fracture of the tibia indicated the possibility of rotation. He stated that an intact fibula with intact ligaments would decrease the extremes of rotation, but it would not preclude rotation. Dr. Gilíes stated that, in his opinion, defendant violated the standard of care for plaintiff’s *960injury when he failed to recognize and treat the rotational deformity of plaintiff’s foot.
Plaintiff moved to prevent defendant’s expert witness, Dr. Robert Hall, from using a model skeleton in his testimony. Dr. Hall intended to use the model to show that Dr. Feinberg did not alter the foot’s rotation, by showing that if he had, he would have created a gap in the fibula. Plaintiff’s counsel argued that the model was misleading because it did not include the ligaments or other muscle attachments which would keep the fibula intact after rotation. The court allowed Dr. Hall to use the skeleton to describe the results of Dr. Feinberg’s surgery.
Dr. Hall testified that the X rays showed no fracture of the fibula. The intact fibula acted as a splint preventing the tibia from moving, thereby preventing the foot from rotating out of position despite the spiral fracture of the tibia. He could determine from the X rays alone that there had never been any rotation of plaintiff’s foot. Dr. Hall based his opinion in part on the X ray of the fibula:
“[I]f the fíbula is not broken or interrupted — I shouldn’t say not broken. As long as it is aligned, despite the fact it may be broken, we know there is no rotational malalignment because with rotational malalignment, the fibula must also have rotational malalignment.”
In Dr. Hall’s opinion, defendant’s treatment of plaintiff met the standard of care, and there was no evidence of rotational malalignment due to defendant’s care. The X rays taken before and after Dr. Feinberg performed surgery showed no change in bone alignment or rotation of the foot. In Dr. Hall’s opinion, Dr. Feinberg performed the surgery for no reason at all.
Plaintiff sought to recall Dr. Feinberg as a rebuttal witness to point out how the post-surgical X ray showed that he corrected the rotation of the foot and to explain why the model Dr. Hall used was misleading. The trial court denied the motion. Although plaintiff’s attorney failed to mention the model in his formal offer of proof made after the court’s initial ruling, the discussion on the record shows that the court also considered the use of rebuttal to explain why the model was misleading. Following the formal offer of proof, the court repeated its ruling.
In closing argument, defense counsel commented on Dr. Feinberg’s cross-examination testimony that the X rays showed a fracture at the head of the fibula:
“Why did he do that?
*961The reason is that he knows, and [plaintiff’s attorney] knows, and everybody that knows anything about legs and tibias and fibulas [knows] that if this is intact, this can’t turn that’s why. So they had to invent a fracture.
Did you hear plaintiff’s expert Dr. Gill[e]s? *** Did Gill[e]s ever say that was a fracture? Uh-[u]h. Never did.”
Plaintiff contends that the jury’s verdict was against the manifest weight of the evidence. A verdict is against the manifest weight of the evidence if
“it is palpably erroneous and wholly unwarranted [citation], is clearly the result of passion or prejudice [citation], or appears to be arbitrary, unreasonable, and not based upon the evidence [citation]. When considering whether a verdict was contrary to the manifest weight of the evidence, a reviewing court must view the evidence in the light most favorable to the appellee ***.” (Ford v. City of Chicago (1985), 132 Ill. App. 3d 408, 412, 476 N.E.2d 1232, 1236.)
The jury’s verdict should not be set aside if there is sufficient credible evidence to support it, and the jury alone should determine the weight and credibility of conflicting testimony. Moehle v. Chrysler Motors Corp. (1981), 100 Ill. App. 3d 353, 359, 426 N.E.2d 1099, 1103, aff’d (1982), 93 Ill. 2d 299, 443 N.E.2d 575.
In this case, defendant testified that plaintiff’s foot was never rotated out of position; it appeared malrotated because of the way plaintiff walked on it. Dr. Hall corroborated this testimony, and he added that defendant did not violate the standard of care. This evidence is sufficient to support the verdict.
Plaintiff argues that the trial court committed reversible error when it refused to allow plaintiff’s expert, Dr. Gilíes, to testify that he could see an indication on one X ray of a fracture on the head of the fibula and that the presence of that fracture further supported his conclusion that the X rays and all of the other medical evidence indicated that plaintiff’s foot was rotated out of position. The court disallowed the testimony under Supreme Court Rule 220, which provides:
“(1) Upon interrogatory propounded for that purpose, the party retaining or employing an expert witness shall be required to state:
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(ii) his conclusions and opinions and the bases therefor[.]
* * *
* * *
*962(3) A party shall be required to seasonably supplement his answers to interrogatories propounded under this rule as additional information becomes known to the party or his counsel.
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*** To the extent that the facts known or opinions held by an expert have been developed in discovery proceedings through interrogatories, depositions or requests to produce, his direct testimony at trial may not be inconsistent with or go beyond the fair scope of the facts known or opinions disclosed in such discovery proceedings. However, he shall not be prevented from testifying as to facts or opinions on matters regarding which inquiry was not made in the discovery proceedings.” (134 Ill. 2d Rules 220(c), (d).)
In a recent decision interpreting this rule, the court stated:
“The purpose of Rule 220, generally, is to promote the timely and good-faith pretrial disclosure of expert witnesses to prevent surprise and to facilitate pretrial investigation of credentials of the expert and preparation of cross-examination as well as the substance of the testimony of the opposing party’s own expert. [Citations.] When the trial court has imposed sanctions for noncompliance with Rule 220, the trial court’s ruling will not be upset in the absence of a showing that the ruling resulted from an abuse of discretion. [Citation.]
However, each subsection of the rule has a different function. Whereas Rule 220(b) allows a trial court to establish a disclosure schedule so as to disqualify an expert who is not properly disclosed, Rules 220(c) and (d) require timely notification of shifts in an expert’s theory or belief and the limiting of the expert’s testimony accordingly.” Baird v. Adeli (1991), 214 Ill. App. 3d 47, 60-61, 573 N.E.2d 279, 286-87.
In exercising its discretion to impose sanctions, the court must “balance the needs of discovery and resolution of the issues on their merits.” (John Biestek & Associates, Ltd. v. Kelly (1989), 192 Ill. App. 3d 301, 303, 550 N.E.2d 230, 232.) Sanctions as severe as disallowing relevant testimony should be imposed only when noncompliance is unreasonable. (Bluestein v. Upjohn Co. (1981), 102 Ill. App. 3d 672, 679, 430 N.E.2d 580, 586.) Once sanctions are imposed, the sanctioned party has the burden of showing that noncompliance was reasonable. (Leeson v. State Farm Mutual Automo *963 bile Insurance Co. (1989), 190 Ill. App. 3d 359, 368, 546 N.E.2d 782, 788.)
“Pursuant to Supreme Court Rule 220 (107 Ill. 2d R. 220), sanctions imposed for violations of discovery rules must be proportionate to the gravity of the violation. [Citation.] The purpose of a sanction pursuant to Rule 220 is not to punish [citation], but to insure fair discovery and a trial on the merits [citation]. A trial court’s exercise of its broad discretion in imposing sanctions for discovery violations will not be disturbed absent a clear abuse.” Caruso v. Pine Manor Nursing Center (1989), 182 Ill. App. 3d 879, 883, 538 N.E.2d 722.
In Phelps v. O’Malley (1987), 159 Ill. App. 3d 214, 224, 511 N.E.2d 974, 981, the Second District Appellate Court opined that trial courts have narrow discretion over sanctions for violations of Rule 220. In that case the trial court permitted the plaintiff to present testimony of an expert plaintiff retained 14 months before trial, although the plaintiff did not disclose the expert until trial. Although the appellate court argued that Rule 220 narrows the trial court’s discretion, it held that even if the court had the broad discretion pertinent to other discovery rules, by failing to impose sanctions against the plaintiff it abused that discretion. We agree with the standard for review of sanctions imposed under Rule 220 stated in Caruso, but we note that even if Rule 220 narrows the range of the trial court’s discretion over sanctions, the court still must consider the reasonableness of noncompliance and the need for resolution of issues on their merits before imposing sanctions. The trial court’s discretion over discovery sanctions does not overcome the duty of “insuring a fair trial for all litigants.” Noll v. Snap-On-Tools Corp. (1980), 89 Ill. App. 3d 1120, 1123, 412 N.E.2d 1185, 1188.
Here, an occurrence witness testified to evidence which the witness apparently discovered during cross-examination. Dr. Feinberg’s testimony on direct examination pertained only to what he saw and what he did in his treatment of plaintiff. He gave no opinion concerning the propriety of defendant’s treatment of plaintiff. Therefore, he was strictly an occurrence witness, not an expert witness.
“An expert is an individual who, because of education, training or experience, possesses knowledge of a specialized nature beyond that of the average person and who may be expected to render an opinion within his expertise at trial. [Citation.] A treating physician is not necessarily an expert witness. [Citation.] Instead, the testimony of the physician *964may be offered only in regard to factual matters of which the doctor has personal knowledge.” (Atkins v. Thapedi (1988), 166 Ill. App. 3d 471, 475, 519 N.E.2d 1073.)
The doctor
“ ‘whose information was not acquired in preparation for trial but rather because he was an actor or viewer with respect to transactions or occurrences that are part of the subject matter of the lawsuit *** should be treated as an ordinary witness.’ ” Tzystuck v. Chicago Transit Authority (1988), 124 Ill. 2d 226, 235, quoting Fed. R. Civ. R 26(b)(4), Advisory Committee Notes (1970 amendment).
When defense counsel handed Dr. Feinberg some X rays and asked what they showed, he testified that he saw a break in the fibula which he had not previously seen. This interpretation of the X ray was expert testimony, since it was his opinion on a matter not within common knowledge, and it did not concern his treatment of plaintiff. The evidence could not have influenced his treatment of plaintiff since he testified that he was not aware of it when he treated plaintiff. Defendant adduced this expert opinion on cross-examination; plaintiff did not in any way seek expert testimony from Dr. Feinberg on direct. For purposes of considering sanctions against plaintiff for violation of Rule 220, Dr. Feinberg must be regarded strictly as an occurrence witness.
After Dr. Feinberg produced the newly discovered evidence, defendant’s expert testified that the X rays showed no fracture of the fibula and that even if there were a fracture of the fibula, it would not affect the possibility of rotation of the foot.
Plaintiff’s expert, Dr. Gilíes, was not allowed to answer because the trial court found his proposed testimony would violate Rule 220. Plaintiff’s expert would have testified that Dr. Feinberg correctly interpreted the X ray, and the X ray showing the fracture further supported Dr. Gilíes’ expert opinion that plaintiff’s foot was rotated out of position. At trial, as in his deposition, Dr. Gilíes testified that the medical evidence showed that the foot was rotated out of position and that defendant failed to properly treat the malrotation. The expert did not change his basic opinion or beliefs.
“[Wjhere testimony supports the larger theory presented by an expert, is not inconsistent with that theory, and does not present an alternative to it, it should be deemed part of the theory for purposes of [Federal discovery rules].” Weiner v. Kneller (D.C. App. 1989), 557 A.2d 1306, 1311.
See also Dugan v. Weber (1988), 175 Ill. App. 3d 1088, 1099, 530 *965N.E.2d 1007.
Although Dr. Gilíes admitted that he had seen the pertinent X rays prior to his deposition, he had not seen the indication of a fibular fracture until Dr. Feinberg pointed it out at trial. Plaintiff did not know of his need for expert testimony on this matter until trial. (See Engel v. Chicago & North Western Transportation Co. (1989), 186 Ill. App. 3d 522, 542 N.E.2d 729 (trial court did not abuse discretion by permitting plaintiff’s expert to testify concerning exhibits presented by defendant, after all experts were barred under Rule 220, where plaintiff could not have known of need for testimony before seeing exhibits).) Defendant has never argued that plaintiff withheld this information from defendant or plaintiff’s expert in an effort to circumvent discovery rules. In this case, plaintiff has shown that he did not unreasonably fail to notify defendant of this additional basis supporting his opinion.
In Potlach Corp. v. United States (9th Cir. 1982), 679 F.2d 153, 154:
“The Government failed by seven weeks to comply with a discovery deadline set by the district court for the exchange of appraisal reports by expert witnesses. As a sanction for this failure, *** the Government was denied the right to call those experts as trial witnesses or to offer their reports in evidence. Following trial, judgment was rendered against the Government.”
The court of appeals found the delay was not due to circumstances in the government’s control, and the record did not support the district court’s conclusion that the government never intended to comply with discovery orders. The court of appeals concluded that “imposition of the sanction under the circumstances here presented constituted an abuse of discretion.” Potlach, 679 F.2d at 154.
In Ratner v. General Motors Corp. (1990), 241 N.J. Super. 197, 574 A.2d 541, the plaintiff’s expert’s proposed trial testimony exceeded discovery responses, surprising the defendant, and the trial court disallowed the testimony beyond the responses as a discovery sanction. The appellate court reversed, finding that the restriction on testimony severely prejudiced the plaintiff and the record showed that plaintiff had no intent to mislead.
“Where[,] as here, the critical surprise evidence resulted through no fault of either plaintiff or her attorney *** and defendant would suffer no prejudice from an adjournment or mistrial, the remedy of exclusion of this vital evidence cannot be said to have been just or reasonable.” Ratner, 241 N.J. *966Super, at 203, 574 A.2d at 544.
Similarly, in this case the failure to give defendant prior notice of the addition to the basis for the expert’s opinion was not due to plaintiff’s fault or circumstances in plaintiff’s control. The record does not show that defendant would have been prejudiced by presentation of the expert testimony, since defendant was able to respond to the new evidence at trial through its expert’s testimony.
When Dr. Feinberg, the occurrence witness, presented new evidence at trial, surprising both parties, the trial court could have disallowed both parties’ experts from commenting on the evidence, or it could have allowed both parties’ experts to comment, within its discretion. Instead, the trial court allowed one party’s expert to comment on the evidence and barred the other party’s expert from responding, when there was no showing that the party who presented the occurrence witness planned the disclosure at trial of the new evidence or otherwise acted in bad faith in its presentation of the evidence. (See Wilson v. Chicago Transit Authority (1988), 126 Ill. 2d 171, 176, 533 N.E.2d 894, 897.) The trial court did not effectively prevent the jury from considering the evidence of a fibular fracture, as it purported to do. Rather than removing the issue from the case, the court allowed only one party to present its evidence on the issue.
In view of plaintiff’s good faith in response to discovery and the admission of testimony from defendant’s expert contradicting the existence and effect of the fracture to the fíbula, the trial court abused its discretion by disallowing opposing testimony from plaintiff’s expert. The prejudicial effect of the court’s action is highlighted by defense counsel's closing argument, which implied that there was no fracture to the fibula because Dr. Gilíes did not so state, and that plaintiff’s attorney fabricated the testimony of a fibular fracture because without such a fracture there could not have been rotation. The trial court must balance the need for .discovery with the need for resolution of the issues on their merits. “The importance of prompt disposition of cases and compliance with the dictates of [sjupreme [cjourt [r]ule[s] *** cannot be allowed to overshadow the ultimate aim of all courts: the attainment of a just result.” Jack v. Pugeda (1989), 184 Ill. App. 3d 66, 79, 539 N.E.2d 1328, 1337.
We emphasize that we do not here decide how we would resolve the case if the expert witness, rather than an occurrence witness, had discovered the evidence. We also do not decide whether we would reach the same result if the newly discovered evidence, which *967is in the nature of expert evidence, had been improperly adduced on direct examination of the doctor who was presented solely as an occurrence witness. We decide only that, when an occurrence witness presents newly discovered evidence, and the trial court allows one party’s expert to respond to that evidence, the court must allow the other party’s expert to comment on the evidence unless the court finds that that party acted in bad faith in its presentation of the evidence.
Since this case must be remanded for a new trial and the other issues raised on appeal are likely to arise again at the new trial, we will address them briefly here. Plaintiff contends that the trial court committed prejudicial error when it permitted Dr. Hall to use a model of the affected bones in his testimony.
“While the use of models or skeletons to assist in the explanation of X ray findings is within the discretion of the trial court, such demonstrative evidence must help explain some relevant issue in the case.” (Webb v. Angell (1987), 155 Ill. App. 3d 848, 861, 508 N.E.2d 508, 518.)
Moreover, such evidence may be inadmissible if witnesses do not provide an appropriate foundation. Gallick v. Novotney (1984), 124 Ill. App. 3d 756, 759, 464 N.E.2d 846, 848.
In this case, Dr. Hall testified that the skeletal model was “a true and accurate and fair representation of the skeletal anatomy including the fibula and the tibia.” The trial court did not abuse its discretion when it permitted Dr. Hall to use the model in his testimony, especially in view of the court’s “limited right to exclude such tangible evidence as is material and relevant.” Seward v. Griffin (1983), 116 Ill. App. 3d 749, 762, 452 N.E.2d 558, 569.
The trial court refused to allow plaintiff to recall Dr. Feinberg to testify in rebuttal to Dr. Hall’s testimony. Rebuttal evidence is admissible if “it tends to explain, repel, contradict or disprove the evidence of defendant.” Illinois State Toll Highway Authority v. Grand Mandarin Restaurant, Inc. (1989), 189 Ill. App. 3d 355, 364, 544 N.E.2d 1145, 1150.
“[Wjhere a defendant introduces evidence of an affirmative matter in defense or justification, the plaintiff, as a matter of right, is entitled to introduce evidence in rebuttal as to such affirmative matter.” Loftus v. Loftus (1907), 134 Ill. App. 360, 362, quoted in Levenson v. Lake-To-Lake Dairy Cooperative (1979), 76 Ill. App. 3d 526, 539, 394 N.E.2d 1359, 1369.
Here defendant’s expert witness used a model of the affected bones to explain his theory concerning the ineffectiveness of Dr. *968Feinberg’s surgery. Dr. Feinberg was prepared to testify on rebuttal that the model did not accurately represent the structures involved. Under Grand Mandarin, the testimony would have been permissible rebuttal as an explanation of matters raised in Dr. Hall’s testimony. At the new trial, plaintiff should be allowed to present a rebuttal witness to explain the model’s defects. We express an opinion on this issue only because it may arise at the new trial. We will not decide whether the failure to allow Dr. Feinberg’s rebuttal constituted reversible error.
Reversed and remanded.
HARTMAN, P.J., concurs.